Sailfin behavior

[frydaddy]

I've had a sail fin tang for a couple of weeks. Until this afternoon, his behavior has always been normal for a tang. A lot of swimming, a lot of grazing, and eating like a pig. This morning he was completely normal. I put a 1.5" x 1.5" square of seaweed on the veggie clip. 10 minutes later I walked by the tank and it was gone, all the fish/inverts were acting normal. About an hour after that I walk by and the sail fin tang's belly is huge, he turned completely black and he's laying on his side at the bottom of the tank but breathing normally. All other fish/inverts are acting fine. Water parameters are all normal. Is this behavior normal for a sail fin? Did he just eat himself to death? Did he just eat too much and this is how they recuperate? Or am I missing something that could take a fish down that fast? The only disease I've seen take a fish down fast is brook and there are usually obvious symptoms first.