"Hole-in-the-head" disease???

My yellow tang's face is erroded. He had two external bacterial skin infections. At first I thought he had a parasite, which was causing his face to wither and also cause him to itch and rub against the rocks, causing the skin infection. Now the guy at the lfs says he has hole in the head disease...something to do with my tank not being grounded, or vitamin C deficiency? Anyone know what i should do??
 
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