The rule I have found for stocking a tank is two inch of fish per gallon of water....The true adult size of the fish must be used in the calculation for tank stocking. Remember - Larger bodied fish create far more waste, and therefore require more water volume. Fish also need room to swim, some more than others.
A ten-gallon tank filled with gravel, rocks, plants, and an assortment of decorations does not hold ten gallons of water. In reality the water volume is often ten to fifteen percent less than the size of the tank.