Catfish are very popular aquarium pets, but they're specific species. Of course you'll see plecos (sucker fish) in every pet store you go to and you might see corydoras, and if it's a really fancy place you might even find things like salt and pepper catfish. Loaches like clown loaches aren't technically catfish but they are related. If you mean can you go out and catch a baby blue catfish or something and keep it in an aquarium that's a whole different question. Although technically you could I wouldn't recommend it, most American catfish continuously grow... not the best idea. Plus I don't know if there would be legal issues involved with that as they are technically wildlife.