While some will say a hippo will work, I'd probably steer clear unless you get a juvenile and grow it out, but even then it might get too big (they get to 12"). Certain smaller acanthurus will work, such as a powder blue/brown, convict, or mimic, and I'll agree with the others that most zebrasomas and ctenochaetus should do fine, but keep in mind that acanthurus and zebrasomas get somewhat territorial with other tangs after they've been in the tank solo for a while.