I've got one too. They only get to be a couple of inches. I've read that they are algae eaters, not detrivores, but I'm not sure if that means cyano and diatoms too.

Reefcleaners recommended a 30g for this sort of critter, but if it's only an inch you'll be okay with him for a while. If it seems like he's hibernating, trade him to a larger tank before he suffers.