This could be several things..
One, Blue tangs (regal tangs, blue hippos, whatever you want to call them) are omnivours, not herbavours. You should be giving him a little bit of mysis shrimp and green nori. You also need to vary his diet. Don't always feed him the same thing.. This could be the cause.
How big is the tank? If it is under 55gal your tank is way to small and you could be stressing him out. If it is between 55-75 gal tank size is questionable.. This could be the cause..
Do you have any electrical leaks? I have seen fish get burnt from electrical leaks before.. This could be the cause..
What temp is your tank? Has your heater ever stuck on? Once, again your fish may have been burnt..
Hope this helps some.. If I can help any more just let me know...