A major problem facing catfish enthusiasts is that their fish are primarily nocturnal by nature. You can purchase a new catfish, watch it all day long and see no problems in the tank. Catfish do their dirty work after the tank lights go out. In addition, some catfish are predators. They make their living and thus survive by eating other fish.
Most fish are opportunistic feeders, eating anything they come across and can easily swallow.
Predatory catfish, on the other hand, are almost exclusively fish eaters. A good rule of thumb when trying to decide about a catfish for your tank is that all catfish with long barbels are predators, but not all predators have long barbels. Even the ever popular Corydoras species, while not considered a schooling fish, are gregarious by nature. Although this may seem too obvious, ask your aquarium dealer for advice. Give him or her enough information so that they can help you.