Yellow Tangs, any tangs for that matter need more in their diet than just greens! Formula 1 is OK but it should not be the only food fed to any fish. A varied diet can improve the health of any fish. Formula 1, Formula 2, Angel Formula, Brine Shrimp Plus are all good foods for any fish, and most fish will eat them all. My personal fav is Special Formula V.H.P. it seems to beef up scrawny fishes quick and I feed all my new fish this with the addition of fresh algae and seafoods, and the above mentioned formulas. With that said, most marine fish have nighttime coloration, aand sometimes especially when the lights in the tank come on, this can be observed. Also look for the white line/ stripe down the middle of the Yellow Tang is this is present all the time your fish could be stressed by someting and should be carefully watched to determine the cause of the stress!