I have Xenia, daisy polyps, zoa's, shrooms(they do kill by stinging),GSP's, and SPS in my tank too. The only way my SPS get bothered is from encroaching corals. I am in the process of getting my shrooms out of the tank, and maybe most of the GSP, the stuff spreads all over the place and is taking over in areas. I use Cerith and Nassarius snails and Cucumbers to keep the sand clean, they do an excellent job for that, I never have crap in the sand bed(knock on wood). This is one time I would suggest feeding more, maybe same amount or slightly more, just broken up in two or three feedings, so the Tang can get more to eat. Does it have algae to graze on during the day? Do you add any vitamins to your food? When I got mine, about 2 yrs. ago, his belly was a little sunken, took awhile, but he is one of the fattest in the tank now. I only feed once a day now, but before the tank was a FOWLR and I would feed twice a day to keep some aggression in check.
Excellent job, keep up the good work.
Matt