Does the fish look "dry"; no mucus or slime coat? I've seen zebrasoma tangs like this after a rough shipping where pH and ammonia got high. I think they get a compromised slime coat. My guess is a secondary bacterial infection then sets. Maybe hypo leads to the same condition if the fish's coat is sloughed enough and the fish is stressed. Is the tang now at regular salinity and acceptable water parameters? Whats nitrate, phosphate, KH? If any of these are off (high/high/low respectively) it makes me think you need to do more water changes. IF everything is good then consider antibiotics. Aminoglycosides target most of the external marine bacterial infections (gram negative pseudomonas or vibrio) so look at kanamycin neomycin or gentamycin. If you don't have access to those or those don't start to work within 24 hours( Personal experience but don't tell your vet cause they want you to run these for days and days) you can try maracyn two (minocycline), nitrofurazone ( water turns yellow), or sulfa drugs (TMP Sulfa), which are more broad spectrum antibiotics but work a little less quickly/effectively (bacteriostatic vs bacteriocidal).
