good luck if it looks that bad its to far and reversing that doesnt happen to often. i had a baby tang i goubght from the lfs
and a week later i saw the first signs and tried to feed him as much green and soaking food in extra selcon. but it didnt help it when from head to tail with in a month. so i will now leave those tangs to the see i now have a salfin tang and havent had a single problem with him. but if you just turned the light on after its been out for a while they do fad a bit.
good luck with him and i hope yours doesnt turn into what i had and my also after 5 months of being in the tank started beating everthying in the tank up. GOODLUCK!!!!