Schooling fish are fish that swim together in larger groups. They will often change direction as a group, and do not appear to have any leader. In the wild, fish would swim in groups to protect themselves. A preditor is less likely to chase a huge group of fish over a single fish swimming on its own. It is possible that swimming together cuts down on the friction from the water and allows the fish to expend far less energy when they need to swim long distances. Also, food is easier to find in a group than as an individual.
Most fish will school at some point in their lives, but not all fish school as adults. Usually it is smaller fish that choose to school. In the wild, a school of fish will be around a hundred fish, and can sometimes be as many as a thousand fish. In your tank, a group of four to six fish that are of the same species that have a tendency to school will form a comfortable school in your tank.
Barbs, Danios, Loaches, and Tetras are fish that are most comfortable when kept in school, and will not be as healthy if they are not kept in a group. Some larger fish, such as the Silver Dollar, prefer to live in schools. Check with the pet or fish store in your area to find out if your chosen type of fish is a schooling fish. You can also research in the library to find this information. Plan on purchasing no less than four of the fish if the one you choose is a schooling fish.
Just because you have a small tank does not mean that you cannot have a fun and interesting school of fish. The great thing about schooling fish is the majority of them are the smaller species. There are many small species of tetras, such as neons and red line tetras, that look great in schools, and do fine in a small tank. The key to having a school of fish in a small tank is to not overcrowd your tank. Remember, the rule of thumb is one inch of fish per gallon of tank water.
If you want to have a school of fish in your tank, consider specializing in one or two types, especially if your tank is small. This will allow you to have an attractive school without over crowding your tank. Keep in mind that in a small tank you may not be able to observe the behavior of moving as one body. There simply is not enough swimming room for the fish to move as one the way schooling fish do in the wild. But, if you have schooling fish, they will be happier in groups, regardless of their behavior. Schooling fish are certainly possible to keep in a small aquarium, as long as you keep in mind not to over fill your tank. Keep the health of your fish as your number one priority, and you will have happy and healthy pets!