The fish does appear to have HLLE; but is just the obvious and not the most important thing with this fish. (IMO & IME). You said the tang went from "normal" to looking like this in 24 hrs? I think this has been subject to long time neglect, I don't know where you got him. PTs are very tough fish and most fish would never survive the lack of proper nutrition, and who knows what else, that the fish went through previously. The tang looks like its been deprived of a good environment and proper feeding for a long time. If it has ich, I can't tell from the pic, that needs to be be addressed first. Then, a tank with enough room, proper feeding, and hiding spots may save this fish.