Blue tang is very prone to ich... esp in a relatively new tank setup...
Only thing u hav to do is to monitor the water closely ( esp nitrate level to be veri high ) then provide good n nutritious food to increase its immunity...
First thing is to buy garlic guard to provide vitamins... secondly is to vary its diet... try to buy some different varieties of frozen food, defrost, add garlic guard... mix well.. n freeze it again....
Some food to recommend: Spinach, mysis shrimp, krill, Quintet Mix...
Not only good health prevents diseases, u will find ur fish having vibrant colours...
Most importantly, be very patient.... this is a long term thing...