in my opinion, one should look first to the max size of the fish. I always use the 2x3 rule and square inchs per gallon. Basically the min size of a tank should be max size of the fish x 2 for width, and x 3 for length and 1 gallon per sq. inch of fish in the tank. In the case of a hippo tang, 12" long so a 2' deep, and 3' long would be a bear minimum and assuming 5" tall and 12" long 50 gallon min. assuming no other fish.