The "one inch per gallon" rule
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"),
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.
The "one inch per gallon" rule
I think the standard inch per gallon rule (just referring to length) is a fine place to start for beginners stocking small schooling fish. One needn't concern themselves with the cubic volume of neons. Using the rule for its intended purpose, its actually a fairly conservative guideline.
I've come up with my own stocking guideline for schooling fish, which is a modification of the inch per gallon rule. In reality, the actual number of gallons an aquarium is is irrelevant. What's important are the dimensions. While a gallon is a standard volume, it is NOT a standard size. A gallon of water can come in any shape. So, my guideline is this - one foot of tank per inch of fish. So a school of 2 inch fish need at least a 2 foot tank. A school of 4 inch fish need at least a 4 foot tank. And so on and so forth.
And I agree - a 10 inch oscar in a 10 gallon is just silly and shows a persons complete lack of understanding of what the "rule" is about.
Last edited by jaysee; 07-14-2014 at 10:42 PM.
125 - BGK, pictus cats, silver dollars, kissing gouramis, palmas bichir, delhezi bichir
125 - Red severum, bolivian rams, Turquoise rainbows, irian red rainbows, loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, pearl gouramis
90 - Congo tetras, african knife, elephant nose, spotted ctenopoma, upside down cats
QTs - 29, 20H, 20L, 10