The "one inch per gallon" rule

[Midwater]

Most of the world has gone metric, and even the fish don't understand the one one inch per gallon rule.

 

[RadiantSpy]

You nailed it! The inch-per-gallon rule isn't about the length of the fish but the cubic inches it occupies. Common sense and fish behavior play a big role in stocking. Good explanation!

 

[https://www.cichlid-forum]

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.

It cannot be applied to a 10-gallon fish tank.

 

[Gershom]

That is a good interpretation of that “rule “. I like it.
Nevertheless I think fish don’t always see things like we do. As an example, I once kept a pair of red oscars and a pair of red tiger oscars all in a 70 gal tank (plexiglass between pairs). Yes I know that sounds terrible, but it was 40 years ago and I was more enthusiastic than informed.
Anyway, they seemed to like it—both pairs spawned, simultaneously, and I raised fry!