I came home from work today and found my yellow tang had a reddish rash all over the rear of his body and he's swimming kinda funny. He just keeps darting back and forth and then hiding. This morning when I left he was fine and had all of his bright color - now he's pale and rashy. Nothing has changed in my tank - no new additions in the past month or so, and the tang was added about 2 or so months ago. All of the rest of my fish are fine. I last tested the water a week ago and everything was perfect. It was also about a week ago when I did my last water change. I'll test the water again in a few minutes. I have attached a photo....what can I do to help him?!?!?!