Wow, you miss a few hours and look what happens. Anyway, if I may, I will try to get back to the original topic of the sick tang. Nopatience, you never told us SPECIFICALLY what behavior the tang was showing. "Swimming crazy" could mean 500 different things. I have a domino damsel that swims upside down on occasion, and has for 6 months. He is fine healthwise, he just seems to enjoy it. Does that constitute "crazy?"
Also, you didn't tell us whether or not you have been supplumenting the tang's diet with vitamin C enriched foods. If you haven't, then VoodooChild's post about lateral line erosion may have been dead on.
GCVT also has a very good point about the SG in your tank. I have yet to find a source recommending less than 1.022.
Now, if you feel that our advice is worthless, may I suggest that you visit your public library and do some reading. There are a lot of books written by a lot of PhD's out there that know more than we do, and they MIGHT even know more than you.