Hey Joe,
Try adding some Cod Liver Oil to your food (available at Target and most other pharmacies, you may need to ask, sometimes its behind the counter). Its really rich in Vitamin A, D, and Omega fatty acids, which can sometimes be lacking in a fish's diet, since we don't tend to feed whole fish to our little ones, rather just chopped up fleshy parts. Don't add too much though, it might make your skimmer go a little nutto, so start out gradual.
Our Purple tang went lateral in QT quite bad, and I started supplementing with this, and he pretty much made a full recovery.
Keep testing your parameters though, there might be something obscure that's off in your water.
--Christy (too lazy to log out)