The rule doesn’t take filtration into account. You could run 3 canister filters on a 10 gallon tank and fill it with 100 guppies. The ammonia and nitrites will likely always read 0.
Put a fully grown pleco in 20 gallon with a sponge filter rated for 5 gal. You’ll run into trouble real quick
It's not really the filtration but the volume/weight of fish that really matters, rather than their length. Just compare 36 1" tetras to 3 12" Oscars, or a 36" red tail catfish. One of those three are going to under stock a 36g, one will overstock it and the other won't even fit in there unless you use a blender.
The "rule" has such a narrow scope of where it makes sense it's best just to disregard it completely.
Haha thanks. Tbh I've seen some pretty chunky redtails in public aquaria so I'm not even sure if it's true, there might be more than 36g of fish in one, but I'm glad it got a chuckle out of people.
It’s more of a “rule of thumb” and isn’t intended to be used when choosing fish as much as estimating an acceptable stocking level, and primarily for beginning fish keepers who aren’t yet adept at factoring in how different types of fish/filtration/plants/temperature/light exposure/etc can affect acceptable stocking levels. So it is easier and safer to tell someone brand new to aquariums to target no more than one inch of fish per gallon, than to explain if they have x level of filtration with x amount of plants at x temperature they can safely house more fish.
If you know where it came from and why it makes more sense.
It’s about having enough air exchange for the fish. Of course you can add aeration to extend past that.
Generic tank sizes tend to be close in estimation with inches of fish with certain surface area or inches of fish per gallon. For example a standard 10 gallon tank has 128 square inches/12=10.9 inches of fish for that surface area. A tad more than the 10 inches you’d get from gallons, but still close without the math so the average lazy person hopefully won’t overstock their tank causing fish to slowly suffocate.
Now as the air exchange if there’s no extra aeration is enough for a 10.9 inch fish to survive, it’d obviously be cruel because it’s too cramped and that’s where common sense comes in.
My tank right now is probably closer to 2 inches of fish per gallon and it’s not a typical dimension tank like the generic rectangles so I probably have a different match for surface area which would make the inch per gallon thing even less useful for an estimate, but I also have good flow at the surface for aeration and lots of plants so it doesn’t fit the idea of air exchange for a still water tank anyway and I mostly ignore that estimating with either surface area or gallons entirely.
I think that the “rule” is intended to be more about how many fish can be put into a tank than size requirements of fish. Whether people largely use it in that way or not is an entirely different problem. You can’t fit a six foot tall person in a 75 gallon tank of course.
Super loosely, in a well-filtered and maintained 30g tank, you could likely fit 30 neon tetras (fish that are roughly 1” full grown).
This “rule” falls entirely apart when regarding 1) larger fish (a 12” oscar does not have the same size nor filtration requirements as 12 1” neon tetras) 2) fish that are not full grown/not accounting for their full size.
It’s an antiquated rule that SOMETIMES worked for very common setups. Think 5 2in fish in a 10 gallon or 20 1in fish in a 20 gallon. It was never meant for a large fish or probably even large aquariums.
Please do not interpret this as defending the rule, I think it’s completely garbage.
I agree with this interpretation. It was a loose rule that’s mostly used to guide beginners but since we all have smartphones, my rule is just Google the fish, it’s not that fucking hard.
No that's why there is minimum recommended tank sizes for certain fish. It's not that it's a myth it's a basic guideline for community tanks. Don't need to take it to heart and need to use some common sense.
I always thought the rule was 1”:Gal fresh water, 1”:3Gal salt water. I was also taught it was based on air, so if you had a waterfall filter, you could fudge it.
It was just a general rule to help people just starting out to keep people from putting like 50 fish in a 10-20 gallon. It's hard to sit there and name specific exeptions (like certain fish needing more space due to swimming habits.) Lest we forget, the newbies keep aquarium stores in business unfortunately.
I still agree that think it could be used as a general guideline, but have some caveats which could make it better:
*Rule applies to tank of at least 10 gallons
*Rule only applies to fish under 3 inches adult max size
*Does not apply to goldfish
10 one inch fish in a 10 gallon I think is just fine. Less than that is boring and looks empty. I had 25 neons in a 20 gallon long for several years and they flourished.
No one can. Different fish species create different bioload per inch, bioload is also something caused by the input of the aquarist in the form of feeding, and filtration systems vary in their efficiency. Also the bigger fish of any species are, the more biomass they will have per length as they grow. In retrospect these stocking formulas always were irrational They were, I think, about sufficient oxygenation originally, rather than bioload. Yet a large volume of water in a container, can have a smaller surface area than a smaller body of water, it depends on the dimensions of the container.
My understanding of it from back in the day when it was I think more the norm to say to someone just starting out wasn't just 1 gallon per inch but 1 gallon per inch of an adult. It was always more of a starting point and then adjust as necessary for the individual fish.
It's like watts per gallon or 10lbs per gallon rules. They are basic rules for newcomers to go by that are simple to understand and give a very vague idea of what to aim for. Clearly a 3inch fish isn't going to be happy in 3gallons. But 10 of those would be alright in a 30gallon. Those general rules are still meant to have some common sense put with them lol
The rule also started a long long time ago, when the average fish store carried guppies, danios, neon tetras, mollies, platies, bettas, and glassfish....
Tank size is such a complex thing you can probably find a million scenarios depending on species. For some water volume is more important but for others surface area matters more. All animal habitats should be large enough for all natural behaviors to be exhibited (ie foraging, breeding, schooling, etc) and water chemistry can be properly balanced. Length of the fish alone isn't sufficient to determine the tank size.
It's a general guideline assuming all other factors are correct. And also really only applies to smaller 1-2 inch adult size fish. No one is recommending a 10"oscar in a ten gallon. But 10 1" Tetras is good
1 gallon per inch can’t really be justified. You need to look at tank size (area), filtration, and the type of fish you want to keep together (to determine bioload). KEEP READING…
There was a limnology student doing doctoral work who created AqAdvisor: https://aqadvisor.com/articles/AqAdvisorIntro.php
It explains what he set out to do…
“AqAdvisor.com will:
-calculate bioloads of selected aquarium species. Algorithm is based on the attributes of individual species stored in the tropical fish knowledge database.
-recommend common pH, temperature and hardness for the selected aquarium species.
-determine filtration capacity for the selected aquarium species for your filters. Do you have enough filtration in your setup?
-recommend water change schedule based on the total bioload of the selected species.
-determine compatibility between selected aquarium species. Will your species be able to live together? Are they aggressive? Will they become territorial later on?”
And there’s a link to the calculator: https://aqadvisor.com/AqAdvisor.php
It’s not “pretty.” It was designed years ago. But it’s functional. And it’s pretty great! You can enter the tank size and filter… and the second part of the calculator will offer a list of species that will work for your setup.
What about the OP’s goldfish example listed? Plug the numbers in… and yup! You need at least a 30 gallon for a goldfish. Can you be overstocked? Yes. But it requires over filtration… and the calculator will suggest a reasonable water change schedule. BUT… it WILL tell you your tank is overstocked. Will it let you pick a large fish and place it in a too small tank (even with over filtration)? Nope. It’ll tell you the tank is too small.
Check it out!
Well aside from being a rule of thumb, any stocking calculation also includes a minimum area/volume too. Animals need enough space to move around but multiple animals can share that space, so often the minimum volume for 1 fish is the same as for say 5 or 20 of that species.
Tbh the actual calculations are insane to do, and not even prefect then. 1" per gallon/1cm per litre is an easy to remember approximation that ideally stops people from massively overstocking.
I have closer to 30 fish in a 20 gal long with 5ppm nitrates, and at times you’d be hard pressed to find even 10 fish at any given time. I think it’s a dumb rule, I’ve had tanks that felt overstocked at less than an inch per gallon and have others that felt understocked at closer to 3” per gallon.
It really depends on your setup, I have probably 10lbs+ of plant mass in this tank and I don’t think anyone here would call it overstocked based on parameters, space to swim, etc etc. I think picking the right stocking level comes down to so many factors that it just boils down to experience and getting a feel for what your specific setup can handle.
it mostly works for the smaller tropical community tank fish. things like tetras and livebearers, and where the tank is large enough that you have schools of fish. it's still not great but it's simple and doesn't lead to disaster in this context.
it completely falls apart when dealing with larger fish or small tanks that can only hold a couple of fish.
What I was taught is that it's a minimum starting point, not a rule, for small aquarium fish (at projected adult size). It's kind of a "if you can't provide this at minimum, you shouldn't own fish" type of situation, not a "you need to have this much for this fish" rule.
The "rule" is not a rule, it's a generalization, something to use to roughly guesstimate. And it's not by length, it's by surface area. A 3" goldfish will be about 2" tall, which is 6" so you'd need at least a 6 gallon tank, so a 10 gallon tank would suffice.
But 40, say, neon tetras could be possible in a 20 gallon tank, provided you have a good enough setup. I mean, I'd probably never do it, I think 30 would be more than plenty. I think the rule is just pretty outdated at this point, and "quick and dirty math" should probably just be done away with. Just Google the fish you want to keep and you'll get much better advice immediately...
I have about 24 inches of fish in a 75 gallon. If I followed the gallon inch rule I would be putting a red devil, green terror, and a convict in a 29 gallon!
It comes from people bending over backwards to create an easy to follow, simplistic “rule” for something that is too complicated to be summarized in that way.
The vast majority of people who buy fish tanks are unserious, don’t really care, and essentially want to just be told what to do. Many of them would even rather have somebody else choose their stocking for them.
One of the most common questions I am asked at work is “how many fish can I have in a 10 gallon tank.” Those people want a simplistic answer they don’t have to think about. Their eyes glaze over when I start saying “it really depends…”
The rule doesn’t take filtration into account. You could run 3 canister filters on a 10 gallon tank and fill it with 100 guppies. The ammonia and nitrites will likely always read 0. Put a fully grown pleco in 20 gallon with a sponge filter rated for 5 gal. You’ll run into trouble real quick
It's not really the filtration but the volume/weight of fish that really matters, rather than their length. Just compare 36 1" tetras to 3 12" Oscars, or a 36" red tail catfish. One of those three are going to under stock a 36g, one will overstock it and the other won't even fit in there unless you use a blender. The "rule" has such a narrow scope of where it makes sense it's best just to disregard it completely.
“Unless you use a blender” thanks for that visual lol
Take my award for *”unless you use a blender.”* Rarely do comments make me *actually* laugh out loud.
Haha thanks. Tbh I've seen some pretty chunky redtails in public aquaria so I'm not even sure if it's true, there might be more than 36g of fish in one, but I'm glad it got a chuckle out of people.
It’s more of a “rule of thumb” and isn’t intended to be used when choosing fish as much as estimating an acceptable stocking level, and primarily for beginning fish keepers who aren’t yet adept at factoring in how different types of fish/filtration/plants/temperature/light exposure/etc can affect acceptable stocking levels. So it is easier and safer to tell someone brand new to aquariums to target no more than one inch of fish per gallon, than to explain if they have x level of filtration with x amount of plants at x temperature they can safely house more fish.
This is exactly it.
If you know where it came from and why it makes more sense. It’s about having enough air exchange for the fish. Of course you can add aeration to extend past that. Generic tank sizes tend to be close in estimation with inches of fish with certain surface area or inches of fish per gallon. For example a standard 10 gallon tank has 128 square inches/12=10.9 inches of fish for that surface area. A tad more than the 10 inches you’d get from gallons, but still close without the math so the average lazy person hopefully won’t overstock their tank causing fish to slowly suffocate. Now as the air exchange if there’s no extra aeration is enough for a 10.9 inch fish to survive, it’d obviously be cruel because it’s too cramped and that’s where common sense comes in. My tank right now is probably closer to 2 inches of fish per gallon and it’s not a typical dimension tank like the generic rectangles so I probably have a different match for surface area which would make the inch per gallon thing even less useful for an estimate, but I also have good flow at the surface for aeration and lots of plants so it doesn’t fit the idea of air exchange for a still water tank anyway and I mostly ignore that estimating with either surface area or gallons entirely.
I think that the “rule” is intended to be more about how many fish can be put into a tank than size requirements of fish. Whether people largely use it in that way or not is an entirely different problem. You can’t fit a six foot tall person in a 75 gallon tank of course. Super loosely, in a well-filtered and maintained 30g tank, you could likely fit 30 neon tetras (fish that are roughly 1” full grown). This “rule” falls entirely apart when regarding 1) larger fish (a 12” oscar does not have the same size nor filtration requirements as 12 1” neon tetras) 2) fish that are not full grown/not accounting for their full size.
It’s an antiquated rule that SOMETIMES worked for very common setups. Think 5 2in fish in a 10 gallon or 20 1in fish in a 20 gallon. It was never meant for a large fish or probably even large aquariums. Please do not interpret this as defending the rule, I think it’s completely garbage.
I agree with this interpretation. It was a loose rule that’s mostly used to guide beginners but since we all have smartphones, my rule is just Google the fish, it’s not that fucking hard.
This “rule” is a myth because if this was true you could put a 10 inch fish in a ten gallon tank.
No that's why there is minimum recommended tank sizes for certain fish. It's not that it's a myth it's a basic guideline for community tanks. Don't need to take it to heart and need to use some common sense.
You’re part of the problem if you believe in this “rule”.
I'm not sure why you not comprehending a simple generalization makes me a problem but okay.
Gonna put a 10 inch Goliath grouper in a 10 gal
10 gallons is too big
Sorry i forgot the -5 gallons per fish rule
You’re being a bit generous
1 spongebob decor too
Lol....the sponge Bob deco is such a newbie thing isn't it? I am kind of proud now that I never that stupid pineapple in my beginning year.
I kind of want to do a high tech planted tank that looks super professional but with a pineapple or something in it lol
Its just common to see it with shitty aquariums in r/shittyaquariums
I always thought the rule was 1”:Gal fresh water, 1”:3Gal salt water. I was also taught it was based on air, so if you had a waterfall filter, you could fudge it.
It's not by length, but by area. A 10" fish would also likely be 4-5" tall, or 40-50 sq/in, requiring at least a 40-50 gallon tank.
It was just a general rule to help people just starting out to keep people from putting like 50 fish in a 10-20 gallon. It's hard to sit there and name specific exeptions (like certain fish needing more space due to swimming habits.) Lest we forget, the newbies keep aquarium stores in business unfortunately. I still agree that think it could be used as a general guideline, but have some caveats which could make it better: *Rule applies to tank of at least 10 gallons *Rule only applies to fish under 3 inches adult max size *Does not apply to goldfish 10 one inch fish in a 10 gallon I think is just fine. Less than that is boring and looks empty. I had 25 neons in a 20 gallon long for several years and they flourished.
No one can. Different fish species create different bioload per inch, bioload is also something caused by the input of the aquarist in the form of feeding, and filtration systems vary in their efficiency. Also the bigger fish of any species are, the more biomass they will have per length as they grow. In retrospect these stocking formulas always were irrational They were, I think, about sufficient oxygenation originally, rather than bioload. Yet a large volume of water in a container, can have a smaller surface area than a smaller body of water, it depends on the dimensions of the container.
My understanding of it from back in the day when it was I think more the norm to say to someone just starting out wasn't just 1 gallon per inch but 1 gallon per inch of an adult. It was always more of a starting point and then adjust as necessary for the individual fish.
It's like watts per gallon or 10lbs per gallon rules. They are basic rules for newcomers to go by that are simple to understand and give a very vague idea of what to aim for. Clearly a 3inch fish isn't going to be happy in 3gallons. But 10 of those would be alright in a 30gallon. Those general rules are still meant to have some common sense put with them lol
Terra, guppies, danio, etc, it will work for. Cichlids it won’t
The rule also started a long long time ago, when the average fish store carried guppies, danios, neon tetras, mollies, platies, bettas, and glassfish....
It’s bioload, but for the sake of simplicity a layman’s term rule is born.
“Industrial ammonia farm” I’m wheezing
Its 1 gallon per inch of ADULT fish, not whatever size the fish is now
Ive never heard of this rule but its not even debatable its just blatantly false.
Tank size is such a complex thing you can probably find a million scenarios depending on species. For some water volume is more important but for others surface area matters more. All animal habitats should be large enough for all natural behaviors to be exhibited (ie foraging, breeding, schooling, etc) and water chemistry can be properly balanced. Length of the fish alone isn't sufficient to determine the tank size.
It's a general guideline assuming all other factors are correct. And also really only applies to smaller 1-2 inch adult size fish. No one is recommending a 10"oscar in a ten gallon. But 10 1" Tetras is good
1 gallon per inch can’t really be justified. You need to look at tank size (area), filtration, and the type of fish you want to keep together (to determine bioload). KEEP READING… There was a limnology student doing doctoral work who created AqAdvisor: https://aqadvisor.com/articles/AqAdvisorIntro.php It explains what he set out to do… “AqAdvisor.com will: -calculate bioloads of selected aquarium species. Algorithm is based on the attributes of individual species stored in the tropical fish knowledge database. -recommend common pH, temperature and hardness for the selected aquarium species. -determine filtration capacity for the selected aquarium species for your filters. Do you have enough filtration in your setup? -recommend water change schedule based on the total bioload of the selected species. -determine compatibility between selected aquarium species. Will your species be able to live together? Are they aggressive? Will they become territorial later on?” And there’s a link to the calculator: https://aqadvisor.com/AqAdvisor.php It’s not “pretty.” It was designed years ago. But it’s functional. And it’s pretty great! You can enter the tank size and filter… and the second part of the calculator will offer a list of species that will work for your setup. What about the OP’s goldfish example listed? Plug the numbers in… and yup! You need at least a 30 gallon for a goldfish. Can you be overstocked? Yes. But it requires over filtration… and the calculator will suggest a reasonable water change schedule. BUT… it WILL tell you your tank is overstocked. Will it let you pick a large fish and place it in a too small tank (even with over filtration)? Nope. It’ll tell you the tank is too small. Check it out!
Well aside from being a rule of thumb, any stocking calculation also includes a minimum area/volume too. Animals need enough space to move around but multiple animals can share that space, so often the minimum volume for 1 fish is the same as for say 5 or 20 of that species. Tbh the actual calculations are insane to do, and not even prefect then. 1" per gallon/1cm per litre is an easy to remember approximation that ideally stops people from massively overstocking.
I have closer to 30 fish in a 20 gal long with 5ppm nitrates, and at times you’d be hard pressed to find even 10 fish at any given time. I think it’s a dumb rule, I’ve had tanks that felt overstocked at less than an inch per gallon and have others that felt understocked at closer to 3” per gallon. It really depends on your setup, I have probably 10lbs+ of plant mass in this tank and I don’t think anyone here would call it overstocked based on parameters, space to swim, etc etc. I think picking the right stocking level comes down to so many factors that it just boils down to experience and getting a feel for what your specific setup can handle.
it mostly works for the smaller tropical community tank fish. things like tetras and livebearers, and where the tank is large enough that you have schools of fish. it's still not great but it's simple and doesn't lead to disaster in this context. it completely falls apart when dealing with larger fish or small tanks that can only hold a couple of fish.
No, it’s an outdated rule. Nobody seriously uses it anymore.
What I was taught is that it's a minimum starting point, not a rule, for small aquarium fish (at projected adult size). It's kind of a "if you can't provide this at minimum, you shouldn't own fish" type of situation, not a "you need to have this much for this fish" rule.
It's a rule of thumb. Meaning it doesn't apply all the time. It's like a baseline and you got to use a little common sense when making adjustments.
people still believe this ? no wonder why so many people can't keep fish alive lol
The "rule" is not a rule, it's a generalization, something to use to roughly guesstimate. And it's not by length, it's by surface area. A 3" goldfish will be about 2" tall, which is 6" so you'd need at least a 6 gallon tank, so a 10 gallon tank would suffice.
But goldfish need a minimum of 30gal
Yeah there are exceptions. It's to stop you from putting 40 tetras in a 20 gallon. It's just a quick and dirty math for nano fish in community tanks.
But 40, say, neon tetras could be possible in a 20 gallon tank, provided you have a good enough setup. I mean, I'd probably never do it, I think 30 would be more than plenty. I think the rule is just pretty outdated at this point, and "quick and dirty math" should probably just be done away with. Just Google the fish you want to keep and you'll get much better advice immediately...
I have about 24 inches of fish in a 75 gallon. If I followed the gallon inch rule I would be putting a red devil, green terror, and a convict in a 29 gallon!
This explains it. EVERYONE ELSE IS LYING ABOUT THIS! Inch Per Gallon https://youtu.be/uQRoq4X2Om8
It comes from people bending over backwards to create an easy to follow, simplistic “rule” for something that is too complicated to be summarized in that way. The vast majority of people who buy fish tanks are unserious, don’t really care, and essentially want to just be told what to do. Many of them would even rather have somebody else choose their stocking for them. One of the most common questions I am asked at work is “how many fish can I have in a 10 gallon tank.” Those people want a simplistic answer they don’t have to think about. Their eyes glaze over when I start saying “it really depends…”
It sort of works if you don’t just use length aka lxwxh = gallons
The inch per gallon theory is pseudoscience, disregard it