Tangs and copper do not mix well as a rule. In future if needing to treat for C. irritans
opt for hyposalinity. If a copper med is still needed, then look for Cupramine
by Seachem. Much better tolerated by a wide variety of copper sensitive fish. Also be sure to get the corresponding multi-copper test kit by Seachem. The ammonia alert badge is also a +
Sounds like it's either bacterial or possibley a true fungal infection but I would lean towrds bactrerial (vibrio). The scales of the fish are most likely standing up making it look much worse. If the other fish have not shown any signs, most likely they won't.
Since the tang has already been through a rough time I'd hate to see it go back in the QT
but I think you should and treat it with Maracyn II at double the dose daily for seven full days. It will also help to soak the foods in the Maracyn. Remove some QT
water to predissolve the med and then take a few tspn of that to soak the foods in. Be sure that you keep a close eye on water quality and medicate any change water before use. Make sure there is no copper remaining from the last treatment and no carbon.
If no Maracyn II is available then Nitrofurazone (Furacyn) at 30mg/gal
for 3 days.