Hey I know they will resolve the situation for you but in the mean time my advice is, check your water parms, monitor the fish to see its behavior and how the other fish react to it, and also try the following foods as they all work for me, and i have about 10 tangs myself
Ocean Nutrition green seaweed very good
Spectrum new life pellets
HIkari marine A
Ocean nutrition Formula 1
mysis
I am sure they did not purposely send you a sick fish, sometimes fish don't do well during shipping or don't accimilate to a tank well.
I had a coral come in this past friday that was about 50% dead when I opened up the bag but everything else they sent me was fine. But it must have just been strssed from shipping, and there is nothing that can be done in that regard.
By the way you should not post personal info like your order number for your own sake, you should PM or email that type of info IMO.