One difference is that a Tang has a pointy nose. The surgeon fish does not. A hepitis or powder blue is actually a surgeon fish. A yellow tang, a Dejardine Tang, Purple Tang are actual tangs. So if someone ask you the difference....
I've always thought the terms surgeonfish and tangs were interchangeable; it refers to the sharp little "tang" that can cut you like a knife when they get ticked off at you when you clean the tank, lol.
P. hepatus is the Hippo or Regal tang; A. leucosternon is the Powder Blue.
Did it ring a bell when you try to put two surgeon fish together? Achillies, Hipo, Powder Brown, Powder Blue all have the same shape and usually will not get along unless introduced in pairs. When They are introduced in pairs its still touchey.
Dave, I sent Nikki and Mojo pictures. I do not know how to post them or I would. Rick took lots of pictures. I will work on that.