Believe that's a Hippo tang. They are usually very hardy fish. You could try doing the water change after the lights go out. What are your water parameters? Is it possible that something is stressing the tang a little? I have one in a 180 at an eyecare center who endures the water changes, the kids slapping the tank, and the infrequent feedings of what he likes to eat with no problem. BTW
, what are you feeding yours?