the 1 inch per gallon rule is a good basic rule but has some flaws. What this rule is trying to accomplish is give a figure for a tank that is comfortable for the fish, both in physical space and, more importantly, dissolved oxygen. See a tall tank can have less fish in it because it has less surface area, if there is less surface area, then there is less oxygen for the fish. So the standard is your surface area, lenght x width= area squared. Each fish in a tropical freshwater aquarium require about 12inches squared to live happily. I hope this helps rather than confusing you, I'm not terribly great at explaining it.