Good maintenance and proper feeding are probably the best things you can do right now, as anything major or drastic will put more stress on the tang. A steady, stable environment is as important as anything right now.
But I'm just echoing what others have said before - keep up the garlic and nori, and water quality. I haven't kept tangs (they make ME nervous), but as with other so-called "delicate" fish you have to provide the optimum environment for them to be healthy enough to lick it themselves.
As to your buddies - not your fault. They're responsible for their own behaviour.
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Alan
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