I bought a yellow tang and his fins were all nipped off. Make sure you get a clip with seaweed attached to it and throw that in three times a week for about an hour or two and let him eat that. Also feed well with a good pellet and brine shrimp, mysis shrimp, and krill. I also use plankton my fish seem to like that.
Also how big is your tank? I got mine in a 55 gallon tank and a lot of people say that is too small. Mine has had no issue and all his fins are healed and he is fat and healthy. It will not be his permanent home as I am upgrading to an 180 gallon at the end of the year. I am just saying I wouldn't have your guy in anything smaller at this point and time of it's development. He will need a bigger home sometime in his life.
Just make sure he is not flashing (Rubbing up against rocks to scratch), and make sure he is swimming. Feed him well and he should come out fine. If your tank is smaller than a 55 I would suggest taking the guy back as he needs some room to swim.
also post a pic if you can, might help to determine if anything serious is going on.
Fish Sticks, the best thing you can do with your dead fish.