Sorry to hear that. Best to look for specimens that are active, have no visual dots. spots, holes, or any sign of infection or disease. It's also nice to see them eat at the lfs before your purchase, but it doesn't guarantee their survival. Blue tangs can be very sensitive and ich prone. You might try to find one from a fellow aquarist that is already accustomed to captive life. Did you happen to buy it from aquarium world? They in general, are not very good at keeping healthy specimens IMO. Use the credit for dry goods and buy your fish from better locations.