Cliff,
My experience with that is that the tang is probably needing to just get fattened up. As soon as you can get him eating I would supplement even daily with seaweed on a clip and he should fatten up. I've had my 5 tangs in my 270 for almost two years and they've not had the wrinkled belly thing going on ever since I got them eating like pigs.
Hope that helps.
Matt