I am guessing it is a Yellow Tang, or a Zebrasoma. They undergo color changes and when that happens they show a distinct white line on each side of their bodies. The cause of their color change can be due to the fact that you had just turned the lights off. Also note that stress can have the same affect on the fish. Just make sure your Tang is not stressed and it will be OK.My tang which I just got today now has a white lines down the middle of him on both sides. They werent there 10 mins ago and just bang white lines? It looks nothing like ich. This was right after I turned the light off. Whats wrong?