Overcrowding is a typical fault. The beginner frequently keeps buying new specimens with out making certain his aquarium will really accommodate them. In a well-balanced aquarium, the number of plants should be roughly proportional to the number of fish. If overcrowding occurs, loss of some of the fish is inevitable.
Tropical fish can stand much more overcrowding than cold-water fish. The latter need 24 sq. in. of surface per inch of fish, but tropicals require only about 8 sq. in. of surface area per fish. This density of fish should not be exceeded.