There is a parasite called black ich that tangs get. It usually isn't fatal. It's a member of the flatworm family. Give the fish a freshwater dip, then a formalin bath, then another freshwater dip, and move to a hospital tank with copper. Wait out the life cycle for three weeks and return to the display. I posted some QT & HT tips here, along with some dip and bath guidelines. I hope it helps.
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