Aloha...may I give you my input on this? This is a very common reef fish in Hawaii found on almost every ledge in 20-40 feet of water Its called the Lavender Tang......(Acanthurus nigrofuscus) They are quite hardy Tangs who are active grazers of Algae. you will find that the yellow color on top is not present among smaller ones....only the larger mature adults have them....aloha Les