"1 inch of fish per gallon of water" Rule
This is a good rule, but has some minor flaws. Its one inch of fish per gallon of full grown fish per gallon of water. If you don't prepare for when the fish grows larger, then your fish will probably outgrow your tank. Also, some fish, such as Angelfish, have long fins, which are very delicate. Angelfish need a taller tank than the average fish for that reason. Other fish, such as Plecostomus, may need a longer aquarium.