The local Mobile veterinarian was explaining to me the other day about keeping fish as pets. Fish are available in pet shops because they are small. They aren't always suited to each other's company. And they don't always stay small. To avoid problems, you must do the following things.
Almost all the fast moving pretty little fish live in schools. Try netting five of them at once, instead of focusing on one individual, and you'll know how effectively this strategy messes up predators. In the wild, a lone schooling fish is as good as dead but alone in your tank it thinks it's done for, will become morose and often die from anxiety. This is why you should never buy one little schooling fish. You will always need at least six. This can be a problem in a 10 gallon tank that only holds ten or twelve small fish. It's another reason why a larger tank is better. Some schooling fish live in busy hierarchies. The popular tiger barb spends its life fighting for rank, not harming its fellow schoolmates, who know the rules. Put one alone, and it will try to train its tank buddies by nipping their fins. When they give the "wrong" response, they get slowly nipped to shreds. A big group of tiger barbs will be so busy smacking each other they often won't bother their tank buddies.
One of the most surprising schooling fish is the Corydoras catfish. Alone, they sit. In schools, they are dynamic, fun creatures. They're happy. Overcrowded tanks lead to diseases and heavy maintenance work. Our local Mobile veterinarian has a number of fish tanks in his clinic.