As has been suggested before, you might want to look at your stocking levels and what you are feeding each of the many species that are in that tank. I see in yet another thread you're now asking about adding a cowfish to this mix. We've gone thru 2 Tang posts, a Angel post, a cowfish post. All asking the same question and ending with the same answer.
55 gallon saltwater tank with these fish: A Juvenile Yellow tang, kole tang, regal tang, porcupine puffer, niger trigger, pair of clowns, damsel, pearlscale butterfly, and a striped blenny.