10 years.... nice. This could be great or perhaps horrible, depending on the evolution of the environment. Most Tangs are very hardy fish, but are usually one of the first fish to show symptoms when a problem hits. What type of Tang is this? How long have you owned this fish? Can you post pictures?
For a tank set up of 10 years, water test results are critical. Many parameters can crash quickly in established systems, especially systems which have been running for many years. I would personally be testing for alkalinity, calcium, pH, and Nitrate. Can you post test results?