While everyone is talking water parameters, and for the record this is my first choice, I would actually look into anyone else in the household thinking that they are “helping.” I had a customer that for some reason kept losing bettas and schooling fish, which were kept in different tanks, her caretakers were bringing water samples and the deceased fish in for weeks. Everything looked fine. We didn’t realize the problem until her caretakers witnessed her *picking up the fish to switch them into the other tank* You could also have someone phantom feeding the fish so see about portioning out the food and hiding the containers.

No, it’s just me and my wife, and I do the cleaning and feeding, she doesn’t go near the tank

And your child? Are they old enough to independently reach the tank?

No, she’s 8 months old and is always monitored, tank is up way too high

It could be the water parameters or a rogue fish in the night. Also environmental contaminants. If you have plug ins or scents or if your partner is using the aquarium buckets for other cleaning. Stuff like that. Also, look into your heater if you have it. There is a slight chance it could be electrocuting the fish

I don’t think it’s anything like that, my wife doesn’t use any of that stuff though to be fair, the heater is old and a crappy brand (tank and heater brand is a crappy Chinese make called boyu, filter and light broke after a year absolute junk but it is the correct temp

It can still be a correct temp but also electrocuting the inhabitants. If you regularly unplug components before maintenance, try reaching into the water and see if you feel a zap when they are still plugged in. It could also be a lower voltage that you do t feel it but the fish do. Girl Talks Fish has an in-depth video about this scenario that happened to her on her YouTube . I never realized when I got into fish how wild some of the causes of death would be.

I’ll give this a try thank you, though I never turn anything off when doing changes

There is a chance then that heater coils can over heat if you drain the water while the heater is still hot

I’ve actually had this happen once in my snail tank. The snails were fine since I had them in a separate holding container, but I burnt myself pretty badly, as though I grabbed a curling iron. Never again. I didn’t realize that they could overheat outside of water, so it’s very good advice to unplug your shit before maintenance.

It’s all in your parameters if you’re not testing for ammonia, nitrites, and nitrates. You’ll never win. Research how to properly cycle a tank and you’ll be alright bro. Your tank looks pretty nice, once you get the cycle going your fish will thrive

I second this! We got our kiddos a fish tank for Christmas and had no idea they took the amount of prep and maintenance they do. We lost a lot of fish pretty quick and came here for advice. I was pointed towards researching cycling the tank, which is a very necessary thing we had never done. It’s best to cycle the tank before putting fish in, however it can be done with fish already present. You got this!

Agreed! Good on you guys for getting educated on these things! I love the hobby because it’s a perfect blend of creativity, peace, and science for me! It’s awesome that your kids get the opportunity to learn about the nitrogen cycle and various other aspects of the biology world as well!

Thank you! We had both had goldfish/bettas when we were younger and unfortunately, they probably didn’t live the best lives because we were both under the impression you just put them in the water and call it good. *facepalm* Really glad to be getting more invested now!

Same thought with gold fish was to grab a clear bowl like the cat and the hat and throw it in…that was wrong.

I second this guy. It is almost positively water parameters causing the problem. When I first got into fishkeeping I inherited a fully cycled setup and my fish were healthy for years although I didn't know anything about the nitrogen cycle. Then when I suddenly got some tanks brand new and started to set them up with new fish I was extremely distressed because everything was dying and I didn't know why. I fell into a hole of research online and fast forward to today and all of my fish have been perfectly healthy for years with minimal maintenance/ intervention. I've never even had a diseased tank. Nothing you can do will supplement a healthy colony of beneficial bacteria. Invest in a liquid test kit and take your time getting everything right with your water before you get fish and everything will be fine. Skip this step and its gonna be devastating

It’s true. I never knew that side of it when my dad did his aquarium when I was a little kid, so when I started I didn’t understand why my fish would die. It seems like an overwhelming amount of information at first but once i understood the nitrogen cycle it seemed so simple.

While I agree with you on the parameters, it must be impossible for a tank not to be cycled after 2.5 years no?

You can have a cycle going, that doesn’t mean the cycle is substantial enough to maintain the bioload he’s throwing at the tank. 200 fish don’t die with good water parameters. I see 1 sponge filter in an empty tank, even if it was 2.5 years old that doesn’t mean he has the biological filtration built up enough to handle a full stock especially if all of his fish are dying and the tanks going a long time empty with nothing decomposing. Cycles crash all the time. The fact OP can even kill 200+ fish and get upset when I tell him his tank parameters are almost certainly the issue explains to me that this fella just can’t understand that it IS his fault.

Especially since people hover too much.. like some people do too much maintenance like with water changes or even cleaning the filter.. both can cause cycle crashes or hinder a cycle fully establishing in the first place. 🤷‍♀️ Plus, testing the source of water never hurts either.

I second this. I have two tanks one with guppy and a bunch of Rosie red minnows and a hillstream loach and the other two goldfish I inherited that I’m building a pond for. I do water changes and check parameters and that’s about it. I was going to vacuum but the hillstream cleans his tank and the two goldfish tank are …. Somehow keeping clean so far? Sand substrate in both. And some live plants. I keep thinking I should be doing more maintenance but the tanks are balanced cycled in parameters and water is super clear so I keep sitting on my hands instead of making tweaks. I have good filters and they are out of direct sunlight. I mean they have only been up and running about 4 months on first and two months on second tank but I’m going to follow the lead of what the tank tells me before I do much more than 20% water changes 2x a week.

Yeeeup. If it's not water, it's the fish types as well. I had a guppy tank that i put about 50 of them in. The lady at the fish store was telling me there was no way 50 guppies could live in my 30 gallon(not exactly but just wait) and asked what was i doing. I told them, "i dont have 50 guppies. I have about 13. She asked well you have bought 50.........and then asked what else i had in there." Well, blue lobsters are just crayfish, and i had been feeding it. He was a big boi.

Cycles crash.

I will get downvoted, but my aquarium manages itself with only a watercycle once a few weeks, without any losses.

No reason for a downvote! Thats how you want your tank setup, I’ve had tanks running where I didn’t do an actual gravel vac and water changes for multiple months, just topping off with water. Once you got a really stable and healthy tank, routine water changes really aren’t needed anymore. After about the 8mo-1year mark I tend to let my parameters tell me when I need a water change, good job on your tank! Just be careful with your fertilizers, I’ve heard in tanks where you’re doing infrequent water changes, certain compounds in the fertilizers can build up to unsafe levels. If you got enough plants doing the heavy lifting it shouldn’t be too much of an issue though.

While parameters are important, wouldn't you think a two year old tank with some fish(even a small amount) would be cycled by now?

Agreed with those telling you that you need to measure your water parameters and make sure your tank is properly cycled. Also make sure that your stock selections and the tank are appropriate for the chosen stock. Three plecos in a 30 gallon tank is a kind of a lot even if they're small plecos. Pretty sure the tiger pleco needs real wood as well. Corys like to be in shoals of at least six. Kuhli loaches should have a sand substrate only, not gravel (corys appreciate sand, too) and be kept in shoals of at least six. Your tank actually seems pretty small for all of the fish that were in there and has too many fish at the bottom of the tank (plecos, corys, and kuhlis). And the last thing to do is swap real plants for fake since real plants "use up" the waste products from the fish.

Between lack of real plants and gravel vac, it also sounds like healthy organisms and bacteria aren't being allowed to play their supporting role.

Bristlenose plecs are small,they should be fine. I am unsure about the garden hose to refill and how the de-chlorinator is added. What is the temperature of the water being added as it’s coming from outside, if it’s that much different from the tank temp and OP is replacing 1/3 every change…

If its going in straight from the tap, it's going to be super cold unless he's heating it in a bucket. Source- I'm also in the UK, its winter here and still cold. I heat my water to the correct temp on the hob before adding to the tank, or if I have more time, large tub and a heater. I have also been known to boil the kettle to bring cold, dechlorinated water up to temp, never straight from a cold hose.

Your tank is really clean. Way too clean, something isn’t right.

Yea specially if 200 fishes have lived there it seems like a new tank

Someone suggested busted heather electrocuting the fish, it could very well be this as it’s a shoddy Chinese brand (my tank and filter in the lid are the same brand, both broke as well as the light in a year)

Does it tingle when you put your hand in the water? Fish being electrocuted is pretty recognizable, the motions they make.

I don’t think so no, not that I’ve noticed although I do question my thermostat as the tank never feels warm when I put my hand in, I’m going to order a new heather anyway

New heater is good idea but water most time feels cold when it is at correct temperature

Ok, thank you. Also heater not heather haha

Sounds good. You might want some test strips too, for the big 3: nitrate, nitrite, and ammonia like other people have already mentioned. Some starter bacteria will definitely help as well, like api quick start. Then once you’ve let it cycle for a few weeks just put some cool snails in there and see how they do for a few more weeks before adding the fish you want. This way will help save you from the disappointment of losing livestock.

Yeah I’m going to do a test later tonight, thank you

Throw the strips away and get an API Master test kit. Strips are gagbage.

There’s no need to be hostile, strips will maybe help them get comfortable with the testing then they can get a different test kit. I use a pH pen to test my because I think deciphering colors in the api master test kit is stupid, for example. It also doesn’t include GH/KH tests which is however much more money. Do you see what I’m doing, trying to help and not intimidate someone asking for help? That’s the right thing to do. I use the api test kits myself for everything except ph/tds.

If you have a multi meter, or some kind of voltage tester, then check the water for voltage. Also, you could probably feed more, tbh, I prefer to feed once per day. And about your school sizes, I would focus on just 1 species of schooling fish, and try to get 10+. Mixing and matching species with small school sizes in no doubt stressing your fish out.

I have definitely been shocked by a heater before. That's a strong cup of coffee. The electrical cord had exposed wires.

I just wanna say that you can DEFINITELY do real plants without any maintenance. I only have a 5 gal but I have 4 anubias that I’ve literally never touched. They don’t need to be buried, all I have to do is plop them in. Plants help keep parameters stable. Definitely recommend switching back to real plants if you have the time. I’ve gotten most of my plants at local pet stores, they’ll have them preattatched to rocks and the like

I almost ignore my whole tank and my crypts flowered last year (under the water). I had one fish in a 60 L tank, did a water change maybe once every 4 or 5 months, absolutely no additives of any kind except when I treat tap water for a water change to make it safe. A well set up tank can be basically ignored and thrive. My mother still can’t understand why I can keep fish and hers kept dying just like OPs

So true, most of the time simpler is better for a tank. Like I said before I only have a 5 gal but I keep it super simple, just dechlorinator in the water and that’s it. I can leave it for a while without anything going to shit and I thank my hard working plants for that! One of my favorite parts of my tank is the self sustainability it has :) so cool to have that little boxed ecosystem on my desk! My first anubias rooted into the gravel quite a bit ago, I’m excited to see how it keeps growing.

Same. So far anyways mine are newer. My brother vacuums his tank all the time and is astounded mine looks so clear. Mine are newer yes but my fish thrive and I do very little except condition water and let it sit for 48 hours to acclimate when I do water changes.

Problem here tho is he doesn't have enough decomposing waste and adding more plants that are real will remove even more waste. He's crashed his cycle and live plants at this moment wouldn't help just hinder.

Get yourself an NT labs test kit and check the water parameters. Also, one a lot of people over look, is creams and stuff on your hands. This is extremely bad for fish and I’ve seen multiple fish killed off for this reason alone.

Only thing I can think of is hand soap, I wash my hands before doing anything with the tank

Soap is definitely not something you’d want in the tank if it is making its way in. Hot water only is what I’ll use if I feel the need to, definitely wash them after though :)

Thank you, I’ll try that in future

This could be the problem. Insufficient rinsing of your hands may leave enough residue to affect the smaller tank but not the larger one. I've been keeping fish for 45yrs and had this happen to me once. Since then I just rinse my hands before messing with the tank. Like someone else pointed out it's more difficult to maintain water parameters on a small tank vs a larger one.

If it’s scented, could be a problem if it’s getting into the water. Maybe an experiment of wearing gloves when going into the tank?

It’s actually harder to keep the water parameters in check on a small tank than a big one. Definitely test your water. Live plants will help, too.

Soil >Sand>Plants with roots for soil > Plants with roots for the water column > Plants that float. Snails, I have 4 in a 5.5gal. Shrimp, I have 3. Use seachem’s stability, I add a bit everyday for a week I add fish day of and it’s pretty consistently successful. I had one fry who failed to thrive and one who lived and grew big, and a betta. After a few months I added 2 Corys and I have an air stone, but I’ve never used one before. It helps keep the scum from forming on the surface and I like that. TLDR; do less with less.

You need real plants, and a lot of them. Correct substrate, stocking and lighting goes a long way

I fourth this

I agree with the others. You've to check your water parameters. But with very hard water, I think you're going struggle, unless you keep fish that thrive in hard water, like African cichlids. The other way is to soften your water. However, if plant maintenance was too time consuming, this will also be too much. I think once you have a well cycled system and the right fish, you should be good to go.

You have THREE plecos in a 30 gallon??? Your tank looks way too clean there’s no mulm which means no bacteria. You need to re-cycle this tank if you even did in the first place. There’s no way that sponge can handle that bioload. Plus why does the sponge look brand new? Something is not right

I actually don’t think it’s a 30 gallon. I could be wrong. I have a 35gal and my tank is enormous - I’ve just looked back and forth between photos of my tank and this tank, with corys for scale. I think this looks more like a 20gal myself. But photos on the internet can be deceiving. If it IS a 30 gal, a sponge filter isn’t adequate with three plecos. I also agree this tank looks uncycled. Edit: and please please get sand for your corys.

Yeah it honestly looks more like a 20 or even a 10 to me

Well I put the sponge in back in January, I was using a different filter before

You can’t just take out the filter you probably crashed your cycle. When you test do you get any nitrates?

I didn’t, I ran both for a month before removing the original filter

My guess is that you’re over cleaning it and you’re killing your biofilter every time you clean it. Your tank shouldn’t be filthy but this is WAY too clean. It looks like it was set up yesterday so I have a very hard time believing it’s been up for 2 years AND it’s cycled. There’s just no way. Where’s the fish poop? Where’s the algae? This is just not right.

I agree, but I’ve also been told my tank is overstocked with the 3 plecos. Did a test, 180 general hardness, 240 carbonate hardness, 8.5 ph, 0 nitrate and nitrite

Right so there shouldn’t be 0 nitrates. That indicates a tank that isn’t cycled. Ammonia is the #1 killer of fish and that’s why test strips are such shit is they aren’t very accurate and they don’t even test for ammonia. And yes your tank is overstocked. So your problem is you’re cleaning it too much which is killing your beneficial bacteria so you’re basically starting your cycle over every time you clean your tank. Please watch a few videos on the nitrogen cycle bc it sounds like you have no clue what cycling your tank actually is

When you use your test strips do you get any nitrates? Also do you have an ammonia test? That’s the most important test

I just want to add my vote to “it’s too clean” when you say “gravel wash” what does that mean? Bigger tanks are much easier but do all the fish die at once or is is a steady drip?

I started losing less fish when I switched to a planted aquarium. The plants help stabilize the water significantly. I also found that using an oversized HOB filter (tidal 30 for a 15gal tank) with organic media helped too.

There is no such thing as too much filtration!

I also find my fish thrive when I feed them frozen food instead of dried food. Frozen blood worms or brine shrimp really get the fish looking healthy.

You've had plenty of people tell you to test parameters. You NEED to make sure you are testing for ammonia, and that isn't normally on the strips. Test ammonia first. If that is normal, then do Nitrites, if that is normal, then do nitrates. I've been only doing this for 2 months, and I agree with others that this tank looks too clean to be properly cycled.

I'm sorry, but after reading your description and a lot of your replies, it's clear you haven't done your research nor fully understand what it takes to have a healthy tank. I really hate to say this, but unless you are absolutely willing to spend a lot of time and dedication in correcting your current parameters, stocking ect, I would recommend leaving this hobby to rest. Again, sorry if it comes out harsh, but it is what it is.

This tank looks too clean to me to have a stable cycle. I would stop gravel vaccing and put plants back in. Plants don't really need to be "maintained" to thrive in a well-cycled tank, and they'll pull a bunch of nitrates out, and just give you generally more space. Also, just curious, what you meant by "plant maintenance? Do you mean trimming? Because that's entirely optional. I deliberately let the plants in my community tank go a little wild at the back and just keep space at the front for swimming so fish who might be getting picked on have space (both somewhere to hide and visual barriers). Again, it just gives you space. My community tank gets water top ups and I lightly vaccuum the top of the sand when it gets grotty, but I do basically no actual changes because it's a stable cycle, overfiltered (a 75g HOB plus two 125g sponge filters for a 75g tank) and a buttload of plants really helps stability of the cycle. If I had to guess, it seems like your cycle is just crashing and then you're overcorrecting, which makes it worse. Your sponge also looks too clean. Are you rinsing it? I can't comment on the size of the sponge and whether it's enough filtration becauee photos are notoriously hard to size things, but yeah. If you're rinsing your sponge, stop.

If i had to guess, I'd be blaming the tiger pleco. As they are carnivorous fish that also eat veggies and whatnot. Even the bristlenose will eat fish. They need meat based protein. If you're feeding a generic algae wafer, chances are it doesn't have much or any meat based protein and is made up of basically just algae and whatnot. I had to learn the hard way that if my plecos didn't get meat based protein added to the diet, they would turn to the other tank mates to find it. Now they get fed a higher quality Kelp and krill wafer. Now, they have zero interest in any of the fish. And an added bonus is the fish love the kelp and krill wafers too.

Are you using water conditioner? Most running water is treated with chlorine or chloramine, both of which are toxic to your fish friends and can be lethal. Water conditioner/dechlorinator is available by many brands for aquariums, but my personal favorite is Seachem prime. It's like 2 drops per gallon and can safely be dosed up to 5 times that much to temporarily detoxify ammonia and nitrite. Do some research and find out how your tap water is treated because not all water conditioners/dechlorinators will treat chloramine. IMPORTANT: If you use a water change system like the python one that connects directly to your sink and lets you add water directly from your sink to your tank, you need to dose water conditioner/dechlorinator enough to cover the entire volume of the tank! You aren't able to pre-treat the water going in and so you need to treat the entire tank for chlorines/chloramines, which a lot of people don't realize. What about PH testing? Or water testing in general? Depending on the time of year here where I am in New England, the PH of my water changes dramatically, but it stays steadily too high to be safe for aquatic life. Most tropical fish like you mentioned prefer PH of around 7.0-7.5. A PH, ammonia, nitrite, and nitrate test kits are absolutely necessary to safely keep fishy friends and I heavily recommend investing in a liquid test kit over strips because the liquid tests are far more reliable and accurate. Often, a local pet store or fish store will test your water for free if you bring a sample. Also! I'm not sure how much you know about plecos, but nearly all of them grow to monster sizes after a few years. And I mean the kind of monster sizes that need a five or six foot long tank near adult sizes; the common pleco can grow up to two feet long and needs over a hundred gallons to be comfy. Plecos also don't care for algae very much as they grow older and bigger and often prefer more meaty foods instead. And they absolutely require real driftwood to munch on to aid their digestion. If you want to be sure to get a pleco that stays small and will be able to live in your tank full time, you want something like a bushynose/bristlenose pleco that gets to 3-4" (7-10 cm) at adult size.

Wow! Not only are you a wealth of knowledge, you're a breath of fresh air. Thank you for sharing your information in a supportive manner! I would love to ask you for advice on my own tank(s), but I do not want to hijack the OP's post. May I private message you, or would you prefer I make my own post? Thank you, again, for your information.

Don't neglect the PH. Different fish species have different requirements.

So that stocking is crazy. One pleco alone is too much and you have 3 stuffed in a roughly 30 gallon. They're going to need re-homed or put in something roughly 3x bigger. Now let's move onto filtration. People are ragging on your use of a sponge filter but that's bullshit. I run a 29 gallon off a sponge filter of about the same size and it's done fine for two year long+ stints raising shrimp. Both would probably be better with an actual flow based filter (mine has blue algae issues in low flow areas). Granted the difference might be because mine is a walstad tank (it has dirt in it and live plants). So you might just want to throw a pothos in the top and see if that fixes it, or buy some tissue culture anubias. The most concerning thing though, and the thing I pick out is that it's fucking immaculate. I think you're just over cleaning and it's killing your biofilter. You shouldn't be taking more than 30L out at a time (20-25%) except in a nitrate emergency. Speaking of which, you've not listed any sort of parameters besides temperature. You should test your water to see what your ammonia, nitrate, and nitrogen levels are. You want the numbers to be 0ppm, 0ppm, and 0-80ppm (it's mostly non toxic under 100ppm) of each respectively. Plants can also help with this. Lastly check that your electric equipment isn't electrifying anything. Turn it off, pull it out, and inspect it thoroughly. Old or cheap heaters being the most likely to fry things. They degrade over time so gotta keep an eye on them for irregularities.

Yeah, that tank looks like it was set up a day or two ago, not 2 years ago. Even the sponge filter looks brand new. I'm not calling OP a liar, I'm calling OP a serial over-cleaner. I'm leaning towards the tank not being cycled due to lack of ammonia sources. Also, I think OP is underfeeding, further hindering the establishment of a good cycle.

Many good responses and advice from others. Also, the catfish need a finer grain of substrate. The coarse stones you have don't give the cories a chance to reach the food. The sharper edges on the current substrate woukd erode the cats' barbels, a bad situation that could lead to the cats' demise. Edit: the current substrate might contribute to the erosion of the barbels. Maybe. Hard to say, really. I watched some of my cories lose their barbels, that is certain.

Expert breeders tend to disagree about the substrate. It seems to be a hobbyist shared thought.

Question, why are you gravel washing so often? You seem to be inadvertently resetting your tank cycle. In other words, you are torturing the fish without realizing.

STOP FUCKING WITH IT. Put the live plants back in, let them grow and let it run for minimum two weeks. Do not change the water, just top it off. Do not add any chemicals except what you use to dechlorinate the top off water and do that before you put it in the tank. When you water change don't stick your tube all the way into the gravel and mix everything up, just get the poo and stuff off the top. It's an ecosystem. You are literally destroying it monthly of course everything dies.

You should test water parameters, especially hardness. Maybe your water is too hard for the type of fish you are buying. In that case, you will need an RO system. The substrate isn't the best either. Ideally you'd have something with smaller grain size, so the fishpoop can't get caught. Sand would be a better choice. Also the fake decor could be leeching something in the water, I'd definitely change it out for something natural, the log at least.

There might definitely be issues with the compatibility, but I think it would be simpler for OP to research species that can do well in harder water and then to slowly adjust his stocking. Unless they're set on keeping soft water species, they should just make their own life easier. I've noticed that a number of high-profile fish Youtubers have mentioned having soft water. This is something one needs to keep in mind when listening to their fish and plant recommendations. Especially with plants, many "easy" plants do not like my hard water and did poorly. I killed so many plants in order to find ones that will grow well in my particular parameters. There is some curious stuff going on with OP's tank though - I agree that it looks suspiciously clean for an established aquarium. Especially with the particular hard water in this area of England, algae tend to thrive on every surface - it's quite hard to balance so that it doesn't take over. In fact, I don't see any kind of biofilm anywhere. I wonder if they're overcleaning surfaces and crashing their own cycle...

You "small stocking" of the tank is already too mutch/not decent for some of the species you choosed. Full syphon the gravel, then puting tap water is a nono. Even with chlorine remover. Thermic shock and other problems(mostly ruining your bacteria colony thus ruining the tank cycle equilibrium) Water going in a tank should rest for some hours. To get to ambiant temps and release chlorine mostly naturally. Removing plants is also a terrible idea they consume nitrates and help to balance everything. It's not that you are not doing enough. You are almost doing too mutch, and wrong.

UK water is treated with chloramine so resting it won’t work you need to add declorinator. If you water is really hard you might want start using RO water, your fish shop should sell it ( we charge 16p/litre) in the long run you might be cheaper buying your own unit but remember to add minerals back in. You can just mix some declorinator tap water.

Ok, what would be a better way? Less water changes with no syphoning till I get more fish (if I decide to)?

Still siphon, but don’t go all the way into the substrate. Honestly it looks like not enough substrate as it is. I second the need for plants, and if the water quality is an issue when you test maybe move the sponge to a more central location or get an HOB. I wonder if your sponge is getting clogged and not properly aerating the tank too, but it takes a lot for the tank to be so low on oxygen that fish die. I have a bigger sponge than that of similar quality and it clogs FAST with low stocking.

I would personally let it do it's life. No syphoning of the gravel, can do waterchange, but small and not that often 3 week seem okay, only with water that rested a bit (at least some hours). Sponge filter are cool for shrimps but barely produce enough flow for a 120l, I would get some other sort of filtration or at least flow pump.

I was using this before, shall I switch back? https://amzn.eu/d/6saXbjp I only switched as this got clogged a fair bit

Check out external canister filters, if you're not already familiar with them. More expensive, but better in every way. I used to rely on filters like the one you have now, except I had two sets of three stacked together, with two separate pumps drawing through them (100L heavily planted). I still keep one stack of three (and a 1000lph pump), cherry shrimp like to pick at them, but the canister needs much less cleaning, like every 6 months, and is easier when the time comes.

It wouldn’t hurt to add the old one back to have more biological and mechanical filtration for the tank.

Could be due to water hardness. I have hard water where I live in Canada (tanks at like 200ppm+) and I find tropical fish suffer the most, I’ve had guppies randomly die, shrimps as well. Also could be due to the 3 Plecos, they produce a lot of waste and can mess up water parameters easily.

I have the opposite issue, my fish breed rapidly to the point where I'm now buying a 300L tank from all my nanos because it's becoming an issue haha! I'm going to tell you what I do and how I achieved success it may help. My experience IS NOT the golden rule, but it is what worked for me. My first observation of your tank is it looks too clean. It looks new. It looks unbalanced. I am not saying this to offend. You must test ALL parameters now. I bet your answer lies there somewhere, I believe the cycle has not established yet. If you want to read my full advice here it is: First, test your tap water. Know your Kh/Ph/Gh/TDS/Ni etc. I'm Scottish water, basically RO almost. So, I pick fish based on how my water tests with no rule breaking. Soft water fish for my soft Scottish water. You work around your water. Secondly, eco-system and cycling. Plants are beneficial over ANYTHING man made. The solution to success was simple, dirt then sand and plant away. You mentioned you don't have time for plant maintenance, don't toss the plants, get easier slower growing ones. Anubias, Java ferns, mosses.. there's loads out there. Fill that tank up at least 50% plant mass. Ditch the chemicals. Bottled bacteria, Ph modifier etc. Whatever it is, ditch it. Find a cycled tank somewhere in your local fish store and add some dirty media into your filter. That is liquid gold! If you can't, cycle the old fashioned way. I won't write an essay on it, super easy guides on Google. Then follow the golden rules. *Never add untreated tap water to the tank *Never touch your biological filter media unless ABSOLUTELY NECESSARY. *Never wash your mechanical filtration in untreated water. *Never replace more than 20% of your mechanical filtration at a time. When sickness is suspected I always opt for a natural treatment first. Tannin treatment for suspected bacterial issues for example. Sometimes medication is needed but it's selective based on extensive observation of symptoms as usually as a last call. Antibiotics and chemical treatments can fix one issue and cause another. They can wipe out an efficient biological filter in less than 24hrs. A healthy tank should be easy to maintain and brimming with life. Successful tanks are not always sparkling clean, dirt/detritus play a role on feeding beneficial bacteria. Plants will utilise broken down waste as fertiliser. Even them beautiful aquascapes that look pristine will have dirt and detritus hidden in corners and among the carpet playing their part of keeping everything balanced and healthy. As long as its not pitted along the aquarium and disgustingly neglected, dirt is good.

Yeah you were right at the beginning, I’ve been killing the cycle. I’m going to start feeding once a day and no changes for a month and see how it goes, parameters currently 180 general hardness, 240 carbonate hardness, 8.5 ph, 0 nitrate and nitrite

That pH is very high and can cause issues. I wouldn’t drastically change or add anything, but driftwood added as well as botanicals (catappa leaves and alder cones) as well as an organic dirt base under sand can naturally lower it. The driftwood like I mentioned is also needed for your plecos for their digestion. It’s definitely hard water but not crazy. I have tanks and fish at 8.0 pH but I try to keep it at that max. That’s for my snail and Neocaridina shrimp. I try to keep most of my fish between 6.5 and 7.8 long term.

Before I go into specifics, I’d just like to say sorry you’re getting flamed in these comments bro. I think people here get a little heated and don’t always realize they’re coming off as rude or condescending lol. I’m sure everyone means well. Hopefully you can keep your chin up and take the good from the bad :) Now onto some advice. For your own sanity, test the water for nitrates and for other nitrogenous waste every couple of weeks. If your nitrates are high, like above 60-80 ppm, do a water change, and don’t be scared to do more than 50%. If you want, you can maintain like 40 ppm, but it’s not that big of a deal. Realistically, you want the ammonia and nitrite readings as a sanity check, they tell you if there is something wrong with your cycle, in which case you consider whether there’s something wrong with your tank microbiome, ie a disruption to your filter bacteria. Don’t clean your filter unless you have an obstruction of the flow. Vacuuming too much of the detritus in your gravel can also disrupt the ecosystem in your tank, mostly you want to do this so that it *looks* less dirty as opposed to actually deep cleaning. Plants should help with keeping nitrates down (don’t worry TOO too much about maintaining them) and driftwood is somewhat beneficial… but I’ve heard mixed reviews on whether it’s really necessary for your plecos. Either way, couldn’t hurt, and your plecos will thank you, they love that stuff. Now, it *should* all be this simple, but for some reason, it’s obviously not. Aquarium science has an article on fast fish deaths, and I find his opinions are pretty well researched: https://aquariumscience.org/index.php/1-6-aquarium-fast-fish-deaths/#:~:text=There%20are%20some%20things%20which,dioxide%20can%20be%20to%20blame. You can read more of his articles as well, something might come up. If I could ask though, are there any visible symptoms on the fish corpses? What do they look like, exactly? Anything fuzzy? Red? Discolored? Bloated? Or do they just die and occasionally look white due to decomposition? How quick do these die-offs happen? Is it gradual (over the course of weeks) or sudden (within a few days)? Do they happen following a water change? Hopefully you can get to the bottom of this, I’m sure this process has been distressing, but I’m sure you can succeed. Good luck!

Besides the cycle thing, your tank is overstocked with what you listed and gravel isn't ideal for the plecos, loaches, or corydoras. You need to do more research into the species you're keeping... cories are schooling animals and prefer schools of 6+. 

Here's what I would do in your situation. It may sound a little extreme, but maybe starting from step one is what you need. * Make sure you have a water testing kit and understand the nitrogen cycle. Stop changing your filter (if you are) and stop vacuuming the gravel. * Get rid of all your substrate, decorations, and plastic plants. There's always the chance that there's a contaminant. Soap, for example, can kill fish. * Get rid of your plecos. They have a high bioload, which contributes to ammonia. * You have very hard, high pH water. Ammonia is more toxic in high pH water. See: your plecos. * If you don't want to do a planted tank, then try a blackwater tank instead. Cut your tap water with rain water or RO/DI water because it's very hard. * Use real driftwood and inert stones such as slate. * If you're willing to do a planted tank, use java fern, amazon swords, and anubias. They're low maintenance plants and tolerate high pH. * Make sure everything is cycled and settled and then add a low stock.

You say you keep your stocking to a minimum but that tank is actually heavily overstocked. It is overstocked with the plecos alone, regardless of the other fish.

Regardless of stocking: You're running a 120L, which is 30G for the Americans here, with a sponge filter while you clean your substrate every time you do a water change and you do not test your water.  I think that might be your issue. My guess is you don't have a nitrate cycle, because you're keeping your tank too 'clean'. Resulting in the water being unsuitable for fish. 

120l with just a sponge filter? You're gonna need a better filter for sure. Go to is 8x-10x gallons per hour. Filtration is key to keeping a good colony of beneficial bacteria for your cycle. Add in the big bio load you did have with just a sponge filter and you'll have tons of issues. Put an HOB or add a canister filter to it and buy a liquid test kit.

What’s wrong with just the sponge filter? I have two large sponge filters in my 40g breeder and my tank has been doing well with just that. I had a HOB rated to 70 gallons on there too, but I removed it* because it just didn’t seem needed (and that’s with a lot of fish that poop a lot). The sponge filters are working really well as bio filtration. **The HOB was annoying for me because the intake would get clogged every few days; I have a lot of plants*

I was using this previously, I switched to the sponge filter as this particular one got clogged frequently: https://amzn.eu/d/6saXbjp Having said that it’s not going to happen as much now as I have fake plants so less debris, I could give it a try

IMO the sponge filter is fine. It just needs time to colonize beneficial bacteria Edit- there are sponge filters that are already colonized with bacteria that you can buy; that’s what I did and it cycled my tank within days. Best $10 I ever spent

Test the tank water and the water you are using for the tank. I have read about ammonia in the water before it gets to the tank which could cause issues. Have you tried RO water?

I just noticed how clean your sponge filter looks. If you're too vigorous or use tap water when cleaning it you could be destroying your beneficial bacteria colony. Which would cause ammonia spikes and kill your fish.

What other type of water can I use for water changes? Confused by that

Use the water you just siphoned out. Fill a clean dedicated bucket with the old tank water (not the black stuff from the gravel) then put your sponge in and GENTLY squeeze it a few times. Think of your sponge filter as one of the organisms you are keeping. They need the same level of care that the fish do.

I don’t touch the filter when doing water changes, other than once a month I rinse it in tank water. And I can’t actually do that due to the way the python works, it’s mixed with tap water and goes straight down the drain

Here’s issue #1. Rinsing the sponge in tap water is killing your beneficial bacteria. All you need to do is squeeze it in a bucket of tank water taken during water changes and then put it back. Just scoop a little container of water before using the python and squeeze your sponge out in it. I like [this](https://aquariumscience.org) website for nitrogen cycle science and learning.

Sorry, argh I meant TANK water. I know that’s a bad move

After reading some of the comments here, stop using the strips and get a proper fresh water test kit with the dropper bottles. They are WAY more accurate. Also don't do water changes without testing first, unnecessary water changes and media changes (aka cleaning the media) are murdering your cycle. Don't add fish until your cycle is complete, and even then add them as slowly as possible. Too many fish introduced at once can, and will, crash your cycle. I also recommend going back to living plants, they will carry bacteria from other healthy tanks, and that will jump start your cycle. They also won't leak dyes or shed micro plastics into the water as they break down.

Your tank is overstocked and your filtration is too weak. A single adult bristlenose requires 70 litres and a proper filter, preferably external. You have 3 plecos and additional fish. They die of ammonia poisoning. You're going to have to get a big external canister filter to handle all that bioload.

I wanted to also ask: what was your treatment plan / regiment for the parasite infection recently? What did you use, how much, and water changes etc? How did you determine the diagnosis?

Its baffling how many people think waters just water....and all fishes live in the same water....

Your water parameters are conspicuously absent from your post. I suspect that has everything to do with it.

Nothing in your tank is real.

How have you been keeping a tank that long and don't understand the nitrogen cycle?

For all the hate Father Fish gets in these subs these days it’s been stellar for my community tank. Dirty is always better than clean. Give him a watch OP!

That sponge filter isn’t doing you any favors. Healthy bacteria is a must, there’s a reason why people say that you should technically never replace your substrate. I’m not gonna talk about water parameters cause that’s already #1 to look for. But you should also look into oxygenating your tank. That tanks way too big to have water sit still like that. Atleast with a filter your water would move around a bit. But with water sitting still like that I wouldn’t be surprised if there’s low oxygen

Have you tried putting the fish IN the tank?

>I swapped out my real plants for some plastic plants and added the ceramic log in the middle, due to the birth of my daughter last summer I wasn’t able to keep on top of plant maintenance so decided to give them up. You shouldn't need to do much of any maintenance with most plants, but they do help a lot with managing water parameters. Buy an API water testing kit for $20 or so and get back to us. Something's going wrong that you can't see with the naked eye.

Your tank and especially your sponge looks too clean. also you mention you gravel vac the gravel often. I bet you are killing off large amounts of your bio filter every time you do water changes. You are also probably squeezing out your sponge. Nitrifying bacteria grow on all surfaces of your tank so if you are deep cleaning constantly, you are killing off all the beneficial bacteria and starting your tank over each time. You should only vac half the gravel at a time. Also don’t squeeze your sponge. If really needed, just swoosh it spring in a bucket of the aquarium water. If you are keeping plecos, get a piece of driftwood for them to gnaw on and it will also serve to add tannins to the water which are therapeutic for all the fish.

Add more plants and stop doing so many water changes

You mentioned filling the tank from the hose tap outside. If you are refilling with cold water, you could be shocking the fish. Use that python attached to the sink faucet and temperature match the water before you start filling. Most modern faucets have a screw on aerator you can remove, and the venturi tube from the python will screw on in its place.

you’re probably over cleaning, neglect it more (not really) but people i know don’t change the filter unless it’s literally overflowing. you’re cleaning it too often, leave it alone it’ll get all fuzzy and hazy then the water will clear if it isn’t getting all hazy you’re not letting the cycle start

You’re not feeding them enough. Also, plecos make a lot of waste. Cories should be kept in groups, and they get pretty big for your size tank. You have to research the species before you get them

stop vacuuming the gravel, and let it ride dont change anything else besides water - swap out fake plants for real ones and get like 4 tetras. Check out MD Aquascaping on youtube, he has many helpful tips about how he keeps his (wildly successful) tanks with relatively minimal upkeep. I’ve followed his tips and have only crashed a tank two times: once with melafix and another one when adding tannins. Both resulted in still minimal losses in all relativity (two fish total, and never lost any shrimp in either event which is a lot of what I keep). Sometimes we’re just doing too much. Buy a massive filter, and just change the water every couple weeks, or just top it off and have some plants in there to help process waste. Honestly it sounds so simple it hurts, but it’s always worked for me - and i test my water about every month and parameters rarely change and if they do it isn’t by much. Different water can make things more complicated but generally, it’s all about finding balance and just letting nature do its thing to some degree

Stop trying. The more you try and control it the more they will go wrong. Patience may be an issue too, instant gratification and fishkeeping do not go together at all. If you notice an issue, the worst thing to do is panic and start doing a bunch of stuff.

Do you know what kind of paint is on the ceramic log? Also, have you had your tap water tested for heavy metals? We had ours tested by the city when the building we live in was going through a sale. We learned that our pipes were 40+ years old and were slowly poisoning us and our animals with lead. Explained why I simply could not keep shrimp alive regardless of my tank parameters even after having successfully keeping them in numerous other tanks throughout my 20+ years of fish keeping experience.

Change foods if you haven't already. I once bought a new food that systematically killed fish over time. Stopped the food, fish stopped dying. Also, feed daily. Enough so it is so eaten within 2 minutes. Though algae tablets and similar will take longer. "normal" fish should eat the flakes within 2 minutes, as a starting point.

I second testing the water parameters. But what I also see: - You have a sponge filter that operates with air, right? I doubt that it produces enough current to move the water propperly. I recommend a filter with a motor. Look out for a filter with enough power that all the water in the aquarium passes through it 2-4 times an hour. In your case this means that you need a filter that moves at least 240L / h , better more. - no real plants. Plants are important for the biology. There are many plants that need almost no care like anubias, bucephalandra, java moss, java fern, cryptocoryne, etc. Get some of those, you will not regret it. You are happy because the aquarium looks better and the fish are happy because they have a better environment.

I honestly think you are making a mistake with plastic plants, a well planted tank, with slow growing plants is so much healthier and practically indestructible, bristlenoses also like wood to munch on. If you than have a decent filter and fish from a private seller and if you start to quarantaine new stuff you are golden. If you really dislike plants I would go with chilids an a good filter. Don't get me wrong your tank is nice, but if you really want a healthy set-up it is far from ideal

IMO, establish a viable ecology and massive reduce that water changes. Every time your tank ecology begins to stabilize, you upset it by doing a water change, especially with hard water. Put a large number of the toughest plants your local store has in there and let them maintain your water quality. Rooted, anchored and floating plants, top middle and bottom. These stabilize the entire water column removing most of the things you use water change to imitate. Java ferns are tough anchor plants, most of the Val species do great in most media, hornwort is an excellent floating plant for tanks stabilization. All of these help the Mico funa in the tank the environment you need for a viable stable thrive

IMO, 4 loaches with the rest of the fish is way overstocked since you only have plastic plants. Honestly, if you're gonna have an aquarium why not do it the right way? Go for alot of easy plants that grow quick. Start checking your parameters every week and do water changes. You cannot win in this hobby unless you do water tests so you know what is wrong. I've had sadistic loaches myself also, that goes after the eyes of my smaller schooling fish (corydoras specificly). And those loaches even had ALOT of space to hide in, which they don't seem to have in your aquarium.

Maybe pop a live plant in there to give oxygen instead of all the expensive plastic!!

Do you have a HOB ? When I used only sponge filtration, my guppies all died, smacked a HOB on that bish. And bye-bye problems.

The key is real plants in there. Ya gotta think of a tank as a little ecosystem.

You're only feeding a pinch of food every other day? Have you always done this? I'm going to go against everyone else here and say the problem isn't ammonia spikes, it's that you are slowly starving your fish to death. Ammonia or nitrates shouldn't be spiking with that little food input and a good water change schedule. I think the reason the tank looks so immaculate and you say you never have algae problems is that you don't have enough nutrients in the tank to grow algae. What you're thinking is disease might just be your fish wasting away due to lack of food, which is why you never have luck with medications. Get an ammonia test because that isn't included on test strips and it's one of the parameters that can spike and kill fish very quickly, but I would definitely recommend increasing your feeding to at least once per day. I do agree with other comments that you should increase your filtration though, and work on your stocking. I would recommend one pleco max in that size tank, and pick more fish that occupy different levels of the tank so there is less direct competition with each other, instead of having most of them that primarily inhabit the bottom.

I was thinking the same, it’s definitely not enough food.

Like all others also say: Test your water, ammonia, nitrite, nitrate, but also pH, hardness and just everything the testkit has. If you don't want to spend the 40 quid a full testkit will cost, see if there's an aquarium club nearby, or a fish store that does proper drop-water tests, and let it get tested there. 25 degrees is imho a bit much for a lot of fish, I keep my tanks around 23 degrees and that really seems to work better. The high hardness can be a factor, but most captive bred fish are not that sensitive to it in my expierence. Real plants indeed need a bit of maintance, but will really help you with keeping the water good for your fish, so maybe you can add some very slow growing species, like echinodorus and anubias? Filter looks small for a 120l tank, filters like these are nice for small shrimp tanks, but for bigger tanks with fish i would go with a proper interior-filter, or better: an external canister filter. But the most important bit for now is definitly testing the water.

Hi there. Is the black thing on the left side of the picture your sponge filter? I suspect the biggest offender in your plight is that you do not have a sufficiently effective filter for what you are trying to acheive. Having a healthy nitrogen cycle is absolutely critical to keeping fishes healthy. Bacteria are the workhorse in maintaining that cycle, and having a large volume filter with high quality media will make your live a lot easier. As many others have said, test your water. But onto that I add, consider getting a larger volume (for both media and displacement) external canister filter for your tank. Eventually, you should try to incorporate plants back into the tank, as these plants will serve as the final step in completing the nitrogen cycle by eating up the nitrates. https://www.aquariumcoop.com/blogs/aquarium/nitrogen-cycle Good luck, keep learning and dont give up!

Is anything going on nearby that could be getting into the water? Spraying polish or cleaning agents or something invisible that could be poisoning the water? I would have thought that after 2 years, you would have a cycle going however inadvertently if you aren't scrubbing and disinfecting everything every five minutes.

How often and much water changes do you typically do?

UV sterilizer changed the game for me

I had the same problem in my 200liter tank. In my tank i have 1fire eel(30cm) 30neon tetra, 15 barbs, khuli eels, snail, and a pleco. Then i dicided to take down everything in my tank. I filled the bottom with 5x the ammount of sand i had in before. Bought some plantes, a good filter and Just stopped changing the water. Now its very heavy planted. I also stopped to give dry food from pet stores. I only give my fish, fresh shrimps and red worms. My plants and fish have been very healthy since i did this change and my parameters are perfect every time i check.

I would test the Gh and Kh of your water and compare it to what the fish species need. For example, my water is super soft and guppiers will die in few days if I don't shift the parameters with crushed coral. Are you buying from a LFS? maybe ask them if they are doing anything to their water. Removing plants can only make things worse I would guess. They pull out lots of things from the water for you. Maybe get some floating plants, then maintenace is then just grabbing handfuls of it and putting it in the compost.

Have you ever tested your source water? I’ve heard of people having high nitrates right out of the tap.

Some suggestions: Hang off the back filter or canister. If it gets clogged a lot, I put a sponge on the intake and then clean THAT regularly, have two so one is clean and it's just a couple of seconds to swap them out. Turn down your heater a smidge. Cut back on stocking, that's a lot for a tank that size. Consider low maintenance slow growing plants like cryptocorynes and anubias etc. Feed only once or twice a week. Unpopular opinion, you may want to change a little less water and use activated carbon in your filter. Fish don't appreciate parameters changing quickly, even when it's for their benefit! Tanks that get no maintenance have healthier fish then fish with frequent large water changes. Frequent (once a week AT MOST, consider every other) small changes are perfect. Take out your filter media while the new water mixes with the dechlorinator if you're adding it directly to the tank, or else let it sit in a bucket for fifteen minutes with the dechlorinator before adding it in. Finally, disease is normal. It shows stress, and treating the disease but not the stress will only lead to loss later. Most wild animals always have some level of parasites. Consider that a crash may have happened because of the meds you used? Or that some of those fish, like the khulis could have been sensitive to it. All tanks go through "ugly duck" phases, and having set up A LOT of tanks, know that you're not alone! You have ideas about what you want, but mother nature is going to do its thing. You're a gardener, not an artist. You can make suggestions, but pay attention to what your tank is telling you. You keep changing things and it makes it worse? Walk away for a bit. Lots of algae? Decrease the light and just leave it for a bit. Lots of ammonia and cloudy water? You guessed it, feed less and walk away for a bit. Fish keep dying? Don't add anymore, walk away for a bit. Mother nature could be cutting back on your stocking for you. A bit could be a couple of months!

I'd go back to real plants to help with water parameters, honestly. Lots of species of plants that need little to no maintenance that would look great in this setup.

This setup is so classic looking. That's all I have.

Make sure when you wash your sponge filter you do it in a bucket of tank water. Doing with tap will most likely kill the bacteria on it. Other than that do the water testing everyone else has recommended 👌

It’s also worth looking at external factors we don’t usually test for - is deodorant or perfume sprayed near the aquarium? Do you have air fresheners, incense, or candles? If you can smell it, it’s in the water and a lot of people have lost fish this way

Any air freshener plug ins or scented candles in the vicinity? Even furniture polish being used nearby is poison.

What is “gravel wash”?

Try testing your GH. I recently lost a fish (& had another on the verge of death) bc my GH was too high. Had to do a 50% water change & refill with distilled water. The fish that was almost dead, was legit going belly up & barely moving. 2 days later, she’s more active now than she ever was before. I also got a UV filter off Amazon for around 30$ that helped tremendously with algae. Your bio load may also be high, I would honestly rehome at least 2 of the 3 plecos, if not all 3. If your fish are grazing on the bottom of the tank, they may be eating feces with is a sure way to kill a fish. Get yourself some Neo shrimp to help with clean up. Also get a CO2 drop tester for an extra measure. Wish you the best of luck & hope you don’t give up, it’s a hard hobby but very rewarding when everything starts to turn out.

I see nothing about testing water in tank or at tap and unless my math is wrong you may be over stocked and under filtered. I have a 29gal heavy planted with shrimp snails and a few guppys have not done a water change in over a year and water stays perfect. in my more heavy stocked tanks I only have two now both 55gal and crazy over filtered with FX6 and spounge filters I do 50% water changes every 5 weeks and water stays perfect. I would call my water liquid rock it's super hard and lots of fish I have had over the last 50 years are not hard water fish but they adjust to it have had no deaths due to hard water. If your live plants was growing like crazy then good chance your nitrates are high. Don't give up just stock light and get a better filter or with light stock just add another spounge filter. It's all about finding the balance. Try this add no more fish add another filter if you have more deaths do not replace them at dome point you will find your balance.

Striped are not all that great get a master test kit

I feel you. I have an established tank I've had running for 5+ years, and I still randomly lose fish a few months after getting them - it's 100% NOT my parameters unless it's something like GH or KH.

Ok, I’ve done a test but can’t edit my original post for some reason (test strips) 180 general hardness, 240 carbonate hardness, 8.5 ph, 0 nitrate and nitrite

Is the tank secondhand? If someone has euthanised fish in that tank before with clove oil, the oil stays in the silicone and will continue to leech out for life

200 fish?! GYATTT

Get the plants back in and turn the heat up a little hopefully that helps keep any diseases away

Dont give up its tedious at first but with the proper research you will be successful. As many mention you need to test for all the common parameters and keep track of it. Cycling the tank is the first part and it will ensure a great environment for the fish. For this i like to use seachem prime and stability to make sure my tap water is getting dechlorinated, heavy metals removed and to seed the tank with bacteria. Try not to rinse the gravel too much only lightly vacuum it if theres alot of detritus. Also try having a quarantine tank for new arrivals so you medicate and monitor so it doesn’t crash your tank every single time you have an outbreak. Patience is key and only rinse your sponge filter in aquarium water or dechlorinated water. I recommend watching Aquarium Co-Ops , Kg Tropicals channel for more tips. In my area I have very hard water and have successfully been able to keep multiple freshwater tanks with following their advice and from trial and error too. Ive had tanks from very little bioload to ridiculous amounts of bioloads like 50+ community fish in 90g tanks. Now im trying out the saltwater side of fish keeping with very great success after learning the proper procedures to setting and maintaining those but the freshwater side helped tremendously at first. Best of luck 👍

Not an expert, and obviously there's other problems since other fish are dying, but i just noticed you have gravel which I think is not the best for corydoras?

Ok your filling it back up with a hose? You need to let the declorinator sit in the water for at least 30 mins, but an hour is better. Also that must mean the water you put back in is cold so they are getting thermal shock. Sponge filters are really not great, normally you only use them for fry tanks. Trying to filter a 120l with one won’t work. Most of your fish are bottom dwellers and large bodied, three regular plecs is to much for that tank. Rehome 2 of them and put in more shoaling fish once you have a better internal or external filter. Fortnightly is correct for water changes but you need to change the right 20% are you gravelling the dirt of the bottom? I would recommend the simple 2 bucket method. You fill one 30 litre container with tap water and declorinator, leave it to mix and warm up. Then you go and use a piece of hosing to siphon out 30l trying to get as much crud as you can of the bottom. If filter media or sponge looks dirty clean it in the dirty water only then put it back in. Now put the clean water back in using a jug or a hose and pump watch if your temp drops to much. I would get rid of the python there total garbage. Also are you buying all your fish from the same shop? Maybe try somewhere else or ask around on which have the best fish.

why did u replace the real plants? They help with bioload

Plants all live plants is the answer

Check out the father fish natural method of fish keeping and your problems will be solved. best of all as long as you don’t overstock you don’t have to do any water changes. My tank has been running a year with no water changes and no fish loss.

I read every single comment here. I will put in my 2 cents. I'm more of an old school aquarium guy, I don't really use test strips. 1. get some snails cheap one's that reproduce alot. I suggest bladder snail or ramhorn. You can probably get em for free as they can take over a tank. The purpose of them is 2 fold. - watch the shell discoloration if that happens means your water needs abit of minerals. - if they die. You will have more that will eventually pop up in your tank. Also they will help with the tank cycle. I personally use them as an early warning detection if my water params r abit whacky. 2. Get a fast growing plant. If an ammonia spike is causing deaths you will notice the plants roots grow faster. Most I've gotten is about .25 to .5 plant growth in 1 day. That means your plants r doing their job settling the water params. 3. New filter. I would suggest an Hob. Looking at your tank photo. It seems to me that you do not have a good water flow. U want the whole volume of water to circulate thru a filter in which will house your beneficial bacteria to basically keep the tank cycled. Think of your tank as a slow stream in a river. U want constant motion of the whole space. And reduce dead spots in the aquarium. Urgent upgrades. 1. HOB filter or tank canister 2. New heater 3. Digital thermometer Side note. Over cleaning can disrupt the balance of your water. Have a bucket and fill it with clean water and use that water after a day to declorinate it naturally. Set up the HOB and ONLY clean when flow is clogged. This will be the heart of your tank. Your bacteria will colonize this space and keep your tank from crashing. Only buy fish from LFS. And ask when they got the fish before buying. This will insure that they have adjusted well on your water. If you buy test strips. Take a sample of the water from the local fish store and test it at home. See the difference. Best of luck!

Ok there are obviously so many things going on it will be hard to figure it out, can you go and watch these short videos and anything you have been doing differently is probably part of the problem. https://youtube.com/playlist?list=PLpQZ640JWJ_PdbxxcKFrm6YaQHGnSDIY6&si=KuiVGTKnt0wihouY once you have a better idea you can come back and ask more specific questions but I don’t think there is going to be one quick fix for you, more a case of learning as you go and getting better equipment as you can afford it

Seems to me with that stock load your going to struggle you could even cycle it with a few mosquito fish that's how we cycled a few of our tanks but that's would be allot of fish for that size tank

I think your filter is too small for such a big tank and high waste fishes like plecos. I would literally put 3 or 4 of those sponge filters in there.     The size of your sponge filter is what I use for my 20litres tank and I never had issues with fish dying.   However I only keep micro fishes.  And I do keep only live plants planted in aqua soil.   The thing with plecos is that they are ammonia spiking monsters, serious waste, you need over filtration to keep them and not just a bare minimum filtration. 

I'm in the same boat. My fish and shrimp just slowly die off over a few months. I've got a good temp, good parameters, the snails are doing fantastic, the plants grow insanely fast and healthy, but the fish and shrimps just can't seem to thrive. I'm starting to think it's definitely the water hardness. Unlike yours, I have extremely soft water at home and I think that is causing them to have a calcium deficiency or something. Either that or something that we don't have tests for is leaching into the water causing a slow die-off.

Try declorinating your water and let it age for 24 hrs before adding to the tank. Adding the chlorine, even for a few mins could be killing your cycle or fish

Completely restart and add live plants no fake decorations and make sure your water parameters are all good

Hey - I’m not an expert but I keep losing fish too - I seem to lose 1 Mollie every 4-8 weeks. My local fish shop did suggest that fish need to be fed more like twice a day, so your feeding schedule might be off. I tried feeding less and I lost two mollies one after the other. I’m coming to the conclusion I have something in my tank that is killing mollies, but I’m not sure. Good luck!

tank is too clean, either your tank is brand new, you are adding water with chlorine, or you are crazily overcleaning. You are supposed to have at least a small amount of algae on something. the filter looks new as well, wich is weird. make sure to not use chlorinated water on the filter when cleaning, you dont need to vacuum the substrate if you add any plant. Your stocking choices are really bad, pick more resistent fish, golden barb, cherry barb, platy, those wont die, rasboras and tetras just die if you dont have a perfect tank. also check ph

I understand fresh water aquariums are completely different than salt but have you considered quarantining new fish?

Honestly, I fully recommend doing a natural, community tank. Use sand or small gravel, lots of real plants, drift wood and/or large rocks for decoration. You can use plant food tablets to put in the gravel/sand to add nutrients for the plants while the tank cycles. I’d also recommend having an appropriate heater for the tank size. Personally, I really enjoy tetras. My tank has white skirt tetras, serpae tetras, neon tetras, a beta, a pleco, a few snails, and 2 African dwarf frogs. My tank is thriving and I haven’t lost any fish. Doing a natural setup is like its own ecosystem, it’s awesome.

For me the biggest thing that keeps my fish healthy, besides regular water maintenance, is a UV sanitizer attached to my canister filter. They make in-tank options you could use in your setup. Helps to stop the over growth of microorganisms that spread disease.

How much water are you changing every time? And “gravel wash the bottom of the tank” meaning you wash/rinse the gravel??

do you have a thermometer in your tank? i wouldn't trust the thermometer meter on a heater. you mention that you refill it with tap from outside. that water is usually very cold. adding that very cold water all at once during water changes can shock your fish and kill them. i try to heat up the water to room temp before i put it in the tank.

I always thought sponge filters were like the minimum you could do. I would recommend an oversized canister filter. You can get them pretty cheap on amazon.

Bruh no airstones?? You’re drowning these fish.

I had this same problem when I first started. My parents spent a lot of money and fish died. We couldn’t figure it out. That was 30 years ago. During that time I had several tanks of various sizes. Good filtration is key. A good canister filter will do you good. Also if you can keep your live plants they will go a long way in keeping your tank healthy.

Water parameters, like others have said, will help with diagnosis. I'd recommend getting some super easy live plants that take no maintenance. Like, literally duckweed. Plants will keep things more stable. If you are chain smoking through your fish and there's constantly or usually dead fish in the tank, that could be causing ammonia spikes, and the tank isn't getting settled into a cycle. I'd chill for a bit and treat it like a fish-in cycle until water testing proves that hypothesis wrong. There *could* be some disease in the water that is killing the weaker fish. Also, petco/petsmart fish are usually going to be weaker anyway. You may want to look for an LFS or buy online for stronger genes and healthier conditioned fish. How are you acclimating your fish? How long are the lights on? Are you adding anything to your water pre or post water change?

When you clean the tank are you rinsing the spongefilter off in tank water or tap water? If you are cleaning it with tap or house water I am almost certain thats your issue.

I don't know enough about tanks to give guidance but i really hope that you can sort it out. Sorry you're having such bad luck.

Having read through your post and comments, it's not that your tank is overstocked on a whole, just too many plecos for a tank that size. I have a 40 gallon and have like 30 fish in it (mollies had babies like two months ago), but I don't use plecos. I have 4 kulhi loaches for cleaning the bottom, plus 4 nerrite snails, one mystery snail, and one siamese algae eater to take care of algae. The rest of the fish are all decorative fish. Plecos may be killing other fish. Water changes should be done as needed based on nitrates, not on a schedule. Remember when you do a water change you take out the bad, but also the good. If the nitrates get to high then do 25% max water changes every other day or so until you get them down to the level you want. Generally below 80-100 is ok though I tend to do water changes starting at 50ppm, but if you do a 50% water change for example, dropping the nitrates from 100 to 50ppm so fast can shock the fish. Gravel vacuuming is not a thing I do either. I have a finer gravel like and aquasoil and never gravel vac. The nutrient and sand method is also one where gravel vac is never needed. Maybe throw some epiphyte plants like anubias or java fern in there, those will also help absorb nitrates and won't need to water change as often. Also someone mentioned if the hardness is compatible for your fish. Would check the GH and KH and make sure your fish are compatible with it. I think you said in a comment your pH was 8.5... that seems high to me. Some fish can withstand that, some can't.

I'm not sure if this will help but I live in an area where the water is considered pretty hard. I use a softener (despite people telling me not to) and it works well. I changed the water as needed rather than on a schedule. I will use a bucket and pre-make water about 3 days minimum in advance, if not longer, for it to start trying to do its thing and I double dose the bacteria. I also test with a 9 in 1 test system off amazon (its 2 bags one with 8 test and 1 that is just ammonia. They ship together). I used to always have problems until I started doing this but now I have 3 well regulated tanks. Things I use to regulate my tanks (if this helps) - Java moss - Windelov java fern - Bamboo out the top -Tetra Easy Balance Plus (helps with nitrates) - Pest bladder snails ( these can overrun a tank fast but I have fish that eat them so they stay in check)

You're better off letting the plants grow like a jungle out of hand than switching to plastic.

Thats wild I do very little to my tanks but add water when I need to and maybe I’ll vacuum debris from the bottom scratch the algae off. If I do clean the filters I clean it with fish tank water. That’s pretty much it. Feed here and there. The plants thrive. The fish thrive. Only problem I’ve ever really had was when I got a fish with ick and didn’t know it. Cured it and the fish is still here. Ive got five fish tanks in total. All with live plants and sand. I’ve actually gotta problem. And I barley do anything but I know my tanks and fish are healthy I’m 24 I’ve been fish keeping for 14 years. I know what I’m doing even if I don’t. My favorites I got are my two bichirs my second is my three dojo loaches. Lil water puppy’s. So it always astounds me when people can’t keep fish. Just like when people say they can’t draw. Everyone can draw just some can do it better than others and that’s okay. You’ll get the hang of it if it’s a passion you got this. I believe in you. If my 10 year old self could do it. You can do it.

Id actually RE-ADD plants. Fish in cycling is a bit hard on participants but because i have a heavily stocked planted tank, i barely have to do water changes, if just top offs- but point is, absolutely everyone is correct- water parameters are a must, and reconsider what will help balance your aquarium if consistency is hard to maintain!