You must have to keep at least one inch fish per gallon of water. You must have to keep the fish depends on the size of the tank. It depends on the size of the tank. The ideal rules to keep the fish in better condition is keep one inch fish in two to three gallon water.
It is important to maintain the health of the fish to get the better management of the water tank. There are different types of fishes available so you need to add the fishes in the tank based on the size of the tank.