Here is a Foureye Butterflyfish (Chaetodon capistratus), though this photo does not do him justice.  I have taken dozens and dozens of photos of these little reef fish, usually managing to capture only their rear end, and still trying to get a decent shot.  This one is overexposed, so it does not show the colors very well, and the camera was at an angle slightly above the fish.  Thus I don't have a true profile view in this shot.  Like the Banded Butterflyfish, these things travel in pairs and flit about very quickly.  It absolutely amazes me that they are so difficult to photograph despite being so common.