These things are all over the reef tops, but practically impossible to photograph.  This is a pair of Banded Butterflyfish (Chaetodon striatus).  Like most species of butterflyfish, they tend to travel in pairs.  They flit about on the reef tops very quickly so that it is very difficult to adjust camera settings, focus, and get a shot.  But that's the challenge!  There are plenty of reef fish (like this one) which I see on every dive but have not been able to photograph.