Tangs are like the idiots that get crammed into houses on reality TV shows - otherwise good folks, but underfed, crammed and overstimulated and they pop. Most people won't feed them enough in hopes of keeping their reefs clean - they need to be fed enough so that they grow an inch or two a year. They need to be fed meat as well as seaweed - they are omnivores, not herbivores. Some quality pellets or mysis will go as far to getting happy tangs as feeding nori. Underfeeding or just feeding nori (without HUFA, vitamin A&E) is also what makes tangs get sick.
FWIW - I have a clown in my 240 and he is a model citizen... well fed, fat and happy. Most people don't believe me, but ask anybody who has been over.
He leaves the anthias and copperband alone even at feeding time and he won't go near the wrasses who stick up for themselves better.