Depends on the tank size and tank mates. Peacock eels are one, there are a few other eels that stay samll. They bury into the substrate and are nocturnal. during the day time you can only see the snout or head sticking out of the substrate. IMHO you will need at least 30 gallons for one of these along with peaceful tank mates.. The substrate needs to be sand that is fine and easy to burrow in.