Had it been eating? If so, what?
As was pointed out, putting Tangs in small tanks will usually stress them. There is not a Tang out there that is meant to go in a 30g. Koles, Tominis and a few others can thrive in a 75g, but most others need 120g or more. The 75g for some Tangs is a recommended minimum size, which I have always read as being okay for the minimum size Tang. Almost every thread you see on here with a "my Tang has problems" comes from a Tang that is in a "too small" tank. IMO
, that is not coincidence.