Hi Ross, Tony is right 100%. Tangs are serious grazers. I have been to Mexico many times and have swam along side schools of powderblue tangs and all they do is eat. I have watched them for hours on end, and seriously they stop long enough to digest and then there back at it again!
I have seen two tangs in captivity that were fat, the first had access to nori 24-7, and the second had some kind of digestive problem. I'm not saying you have a sick fish by no means, instead I would say that you have a very healthy one if all it wants to do is eat.
cheers, Rich