This would be so much easier with a picture of the tang and the affected area.
Could still be a few things. The reason I asked the above was I was thinking HLLE
which would look similar to this:
Considering the other fish at the LFS
had this and the store instructed you in the use of formalin the one thing I am leaning toward at this point is turbellarians
(aka black ich), which if that's the case I would never shop there again.
The other possibility would be flukes. But you should see some sort of texture or "raised" area in either case.
I would definately call the LFS
using your "friendly" voice and find out for sure. Don't get upset until you get an answer or they'll probably just hang up.
I hope the tang is in a QT, if not you may need to QT all the fish in the main display. Please let us know if anything here rings true or an honest answer is forthcoming from the LFS