You could put him in a 40 gallon. It'd give it alot of room, and you could have a nice size pool in there. As long as you don't mind changing the water every other day, if not more often. My only concern would be feeding if you do feed him in the cage. They are ambush predators, and with that big of a cage may cause issues as far as food coming to him. I'd maybe watch the first few feeding's, and as long he's getting fed then you should be fine.