Selecting Your Fish

Stocking your aquarium should be a gradual process. It is best to add only a few fish at a time to avoid overstocking the tank and jeopardizing the health of the fish. No hard and fast rules exist about how many fish you can put in your tank for a given volume of water, but it is better to be understocked than overstocked. A rough estimate is 2.5 centimeters (1 inch) of fish body length per gallon of water, meaning you can put about ten 5-centimeter (2-inch) fish in a 20-gallon tank. This number, however, can be safely doubled to 2 inches per gallon in a well-filtered and well-aerated tank.

Most aquarists start with a so-called community tank, which is a mixture of compatible fish selected without much regard to what parts of the world they come from. There are a few basic rules for picking such a mixture. You should consider the behaviors of the species you will mix, such as their aggressiveness or sense of territory; the level of the tank at which the fish eat (top, middle, or bottom); the size each type of fish may reach as it matures; and the chemical composition of the water you will be using. A knowledgeable pet-store employee can help you pick the right mix of fish for your aquarium and tell you how to introduce them to your tank.