Jon,
Sorry to hear that. Is your tang alone in the tank? If not, it's best to remove him to a quarantine/hospital tank to reduce exposure to other fish in the tank. You may want to do some research on
www.wetwebmedia.com on fish diseases. This site is a wonderful place to go to research health problems on fish. There are many experts here. Although it can be hard to find exactly what you're looking for, I've found lots of great info there.
Good luck with your tang...hope everything turns out ok.
Melissa