I have a 6 yr 55 gal reef tank with 10 fish. No new fish have been added for 3 years. Over the last couple of days I have noticed my yellow tang (in 4 1/2 yrs) has a red rash on back area. What is going wrong? All tests within normal limit and all else healthy (fish, shrimp, hard & soft corals, etc). Love my tang is there anything I can do to help or cure her?