The "one inch per gallon" rule

crayfishlobster

Jack Dempsey
Aug 23, 2017
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I know, but some fish are really active. if u want a happy arowana for life get at least a 375 gallon or better a 400 gallon. I am also talking about the people that think it is only about length.
 
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ozliosoma

Jack Dempsey
Nov 4, 2008
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I have always wondered how this worked. Thanks for everyone taking time to explain.
 

itrebebag99

Plecostomus
Mar 16, 2017
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It's always sensible to gauge the number of fish for a tank by the size they'll likely attain in that aquarium. I don't want to specify any particular number for your 250. But, here's something I hope will help you decide.

View attachment 1247458
Well, lets just be fair here, most orandas (or any other fancy goldfish) won't get half that big. A goldfishes size is almost entirely dependent on environmental conditions, not genetics. A goldfish requires warm water, lots of food, lots of water changes, lots of water changes, and lots of space to have a chance at growing that big. Not exactly reasonable to assume that an oranda will grow that big in a 29 gallon, and outgrow the aquarium. I know this isn't exactly your argument, and is slightly besides the point, just wanted to point this out.
 

Alphonsus

Feeder Fish
Sep 10, 2017
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Sadly people who go off by the inch per gallon are usually people who just want fishes or lots of them and don't care about their actual well being. Of course in every pet hobby you have those people who don't mind keeping a hamster in one of those plastic cage, or a bird in those small barred cages, or a oscar in a 55 gallon tank.

When it comes to stocking there is way more than just the size of the fish. One big part is the activity level of that fish. This is why harlequin rasboras require a 20 gallon tank rather than a 10 gallon tank, size wise a 10 gallon seems to fit but the 10 gallon is too small for them to be able to actually swim around.

As for the max size of fish, let's just say for those bigger fish usually they do get big if you properly care for it. For goldfish, if you get a common goldfish, you can only have a 6 inch or only 10 inch goldfish, this will usually be due to stunted growth and not the proper nutrition. However if you keep them in a pond(how they should be kept) they tend to get to the 15+ size.

This is what separates us aquarium hobbyist. There is a difference between those who truly care about the fish and wants it to thrive compared to those who just want fish to pleasure them. As they say, no fish is "hard to keep" if you fulfill the proper requirements of that fish you shouldn't have a problem.
 

Alphonsus

Feeder Fish
Sep 10, 2017
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Also to add on about the measurements that people were using in the beginning, it's kind of nonsense. It's almost like a stereo type where you treat all fish as the same and thus categorize the minimum size tank by their body volume. Fishes are different from each other, if you have 2 6 inch fish, one is active and the other isn't they will both need different tank size. You might only need a 36" tank for the non active one while a 4 feet tank for the active one. Another thing to consider is also bioload. This is why for most plecos that stay under 6 inches it's recommended for a 20 gallon. Plecos produce big bioloads, thus if kept in a 10 gallon it will foul up the space more quickly.

These factors are what makes stocking more complicated than simply measuring them. You have to understand the behavior of the fish also.
 

Jeditaz

Gambusia
Oct 25, 2017
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10 years later and this post still enlightened me. Thanks Guppy! I never understood the rule until just now.
 
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TUCCI

Gambusia
Apr 13, 2018
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This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".

This is a suggested guideline for a well maintained and filtered tank.
It does not apply to all fish as some have differing requirements.

Here is the part that is being misunderstood.

The "rule" does NOT refer to the length of the fish!

The "rule" applies to the cubic inches of fish in the tank.

This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.

For small fish like glo-light tetras you will end up with something like this,
1 1/2"x 1/4"x1/2", this comes to 3/16 of a gallon (about 1/5), and that gives you 5 fish of this size per gallon (quite reasonable)

For larger fish you end up with something like this, my example here will be a silver arowana at 24" long, 24"x 4"x 1", which gives you 100 gallons of water.

As you can see this works fairly well.

You do also have to apply some common sense and allow for such things as potential growth, the fish types' tolerance for crowding, and of course the width and length of the tank (a 24" gar will not work in an 18" wide tank even if the tank holds 100 gallons).

So please people, accept that this is just a generalized guideline to figure potential stocking levels, not a hard and fast rule.
Also remember that just because you don't like it doesn't mean you should slam somebody for using it.
And lastly, please don't flame someone by saying a 10" oscar doesn't fit in a 10" tank.
Of course it doesn't,
but the rule never said it would.
Well said and thanks for clarifying it.
 

Davidiator

Piranha
Sep 17, 2017
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David
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This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".

This is a suggested guideline for a well maintained and filtered tank.
It does not apply to all fish as some have differing requirements.

Here is the part that is being misunderstood.

The "rule" does NOT refer to the length of the fish!

The "rule" applies to the cubic inches of fish in the tank.

This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.

For small fish like glo-light tetras you will end up with something like this,
1 1/2"x 1/4"x1/2", this comes to 3/16 of a gallon (about 1/5), and that gives you 5 fish of this size per gallon (quite reasonable)

For larger fish you end up with something like this, my example here will be a silver arowana at 24" long, 24"x 4"x 1", which gives you 100 gallons of water.

As you can see this works fairly well.

You do also have to apply some common sense and allow for such things as potential growth, the fish types' tolerance for crowding, and of course the width and length of the tank (a 24" gar will not work in an 18" wide tank even if the tank holds 100 gallons).

So please people, accept that this is just a generalized guideline to figure potential stocking levels, not a hard and fast rule.
Also remember that just because you don't like it doesn't mean you should slam somebody for using it.
And lastly, please don't flame someone by saying a 10" oscar doesn't fit in a 10" tank.
Of course it doesn't,
but the rule never said it would.
So according to this I could have 750 glow light Tetras in my 150?
 

___James___

Exodon
Apr 20, 2018
56
30
21
19
Real Name
James
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Male
Sadly people who go off by the inch per gallon are usually people who just want fishes or lots of them and don't care about their actual well being. Of course in every pet hobby you have those people who don't mind keeping a hamster in one of those plastic cage, or a bird in those small barred cages, or a oscar in a 55 gallon tank.

When it comes to stocking there is way more than just the size of the fish. One big part is the activity level of that fish. This is why harlequin rasboras require a 20 gallon tank rather than a 10 gallon tank, size wise a 10 gallon seems to fit but the 10 gallon is too small for them to be able to actually swim around.

As for the max size of fish, let's just say for those bigger fish usually they do get big if you properly care for it. For goldfish, if you get a common goldfish, you can only have a 6 inch or only 10 inch goldfish, this will usually be due to stunted growth and not the proper nutrition. However if you keep them in a pond(how they should be kept) they tend to get to the 15+ size.

This is what separates us aquarium hobbyist. There is a difference between those who truly care about the fish and wants it to thrive compared to those who just want fish to pleasure them. As they say, no fish is "hard to keep" if you fulfill the proper requirements of that fish you shouldn't have a problem.
Very well said. I agree.
 

claytonyu

Feeder Fish
Apr 19, 2018
6
2
3
20
Real Name
Clayton Yu
Gender
Male
However this doesnt take into consideration the width and height of the fish
 
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