Hi Snugy, welcome!
I checked for your tank specs to find out more and it says that the tank is 12g, is that right? If it is then you should remove the tang from the tank as soon as you can. One of the general guidelines for tangs is a tank with 75g capacity or larger. People successfully keep them in slightly smaller tanks, but without an established, mature tank it can be difficult. The water testing you had done cannot show viruses, bacterias, etc. which are impossible to remove completely anyhow. A healthy, stress free fish is able to fight off many illnesses, the same as healthy people are able to. A stressed fish (either from poor water quality which doesn't sound like the problem, or from cramped quarters, or unnatural environment, or shipping stress, etc.) is simply more prone to coming down with a health problem.
I won't try to diagnose the malady, but I would recommend picking up a good book that goes over illnesses, feeding, maintenance, etc. A few titles would be "The Conscientious Marine Aquarist" by Fenner?, "The New
Marine Aquarium" by Paletta, "Natural Reef Aquariums" by Tullock, and Scott Michaels' "Marine Fish" (I think that's the title) it gives the lowdown on care for about 500 species.
Good luck!!! Let us know more specks if you can (feeding, temp, tankmates, filtration, etc.). Talk to you soon
Again, welcome!
Chris
