[BoomerD]

Nikki, I too doubt that the hyposalinity is the actual cause of the problem. My thought was that bringing the fish back to normal salinity may be less stressful than the hyposaline conditions.
Tangs are very dependent on the "gut fauna" to process their foods, which is why they are not recommended for copper treatment, which may kill those bugs that make it possible for the tang to be the great algae eaters they are.