Stocking Your Aquarium
You are probably ready to get your fish. Before you get any fish you must wait three days to allow your biological filter establish. When you get fish you should only get two fish the first week. After that do not fill your aquarium to the maximum load of fish until six months later.
Most people will say add 1 inch of fish per gallon but this is wrong because for example a 2-inch fish will weigh ⅛th the weight as a 3-inch fish, which has the same shape and size. So the 2-inch fish will use up ⅛th the oxygen as the 3-inch fish and they will both put out the same amount of carbon dioxide and the 2-inch fish will have ⅛th the wastes given off as the 3-inch fish. Now you can see why the other measurements for fish are wrong.
Here is a chart and the equation to help you decide the correct number of fish you can have. You will see that you can have more smaller fish than the 1-inch of fish per gallon rule by following this one but you can have less bigger fish.
X² (2+x) +2
Length of Fish in cm
Space Needed Per Fish in cm