In my experience loss of color was related to stress. Now that could be simply stress or worst, stress has compromised an immune system and your dealing with something else. Like the above post has there been any changes recently? Depending on the rest of the systems needs I go as stress free as possible. Lights out, lots of feeding (if he won't eat that's a symptom we should know about) and as little "hands in the tank" as possible. Make sure, since its a tang, that ita getting ita fair share if nori, veggies, and a little protein