Fish that have different food requirements can make it difficult to supply each species with the appropriate diet. Those that require lots of protein will need different foods than those that require more vegetable matter. Growth and health are tied directly to a nutritionally balanced diet, and the proper balance can vary from one species of fish to another.
Keeping fish of vastly different sizes in the same tank, no matter how peaceful, is an invitation to disaster. Fish will eat whatever fits in their mouth.
Many a novice has been shocked to discover that the angelfish and tetras that coexisted so well when they were small are no longer safe together -- the increasingly larger angels have no qualms about consuming the smaller tetras.
Reading as much as you can about aquarium fish and asking your dealer for advice will help you choose the best combination of fish for your size tank. Keep in mind that descriptions of fish behavior are generalizations, and some individual fish are exceptions to the rules.
When shopping for fish, the first thing to keep in mind is that the dealer's tanks are greatly overstocked. The store tanks receive much more maintenance than home aquariums do, and the fish will only be in those tanks for a relatively short time, perhaps a week or less in many cases.
Also remember that the vast majority of fish sold in stores are very young. You must stock the tank based on the normal adult size that the fish should reach when mature.
This will mean that for the first several months, the tank will appear somewhat empty. Don't fill that space with more fish! The small fish already in the tank can only grow to healthy adult size if there is room.
Healthy fish are active. Avoid fish that have poor color, fins clamped close to the body, or odd swimming behavior.
When you approach the tank, the fish should crowd to the front in anticipation of being fed. If some of the fish in the tank are clearly sick, do not purchase any fish from that tank.
When choosing fish, keep in mind that you don't want to select ones that all live in the same part of the tank. That is, you want a community in which some fish swim near the top of the tank, others in the middle, and some at the bottom. Some species tend to stay at one level most of the time, whereas others are all over the tank.
Learn how to introduce aquarium fish to their new surroundings on the next page.