This topic comes up a lot with tangs. Hippos are one of the hardiest fish we keep and I've seen a lot of live, but very raggedy looking ones. IMO & IME; tangs are not inner reef fish and need a lot of swimming room. A 40 pillar doesn't offer them much. These tanks are only 30", if I remember correctly. A "baby" will be ok for a while; but I just don't think there is any magic number that would tell you when its time to move. Just the sooner the better. I've convinced that tangs that are cramped for long never do reach their full potential. Just my opinion: there are so many of these little hippos available and I think the vast majority of them are never kept in a suitable tank. These fish can reach 12" and even LA suggests a 180 (6' long) as a minimum tank size. Not much of an answer; just want you to know whats ahead.