Do aquarium catfish prefer sand or gravel?

Catfish... aren't picky in the slightest. It wound be like asking a pig if it would rather live on a farm or an orchard. You can have a bare-bottom tank and they will be happy. As long as there bellies are full, it doesn't matter. but keep in mind. Large catfish LITERALLY inhale their food. So if you have large catfish, it be best to have gravel to prevent choking.