I can't find anything about aquariums or tanks, so I would think you'd use the same rules as you do for any outlet. It has to be accessible, not necessarily "readily accessible," it could be in a cabinet or something. It couldn't be considered a damp location, but if you had water spraying down it might create other problems. And I don't think GFI's would apply. They don't care if fish get shocked I guess. Atleast that's my opinion.