I recently read in one of my fish mags where hlle was linked (in tangs) to carbon use. Just like in freshwater discus, the carbon can "strip" the slime coat off the fish which leaves it vulnerable to other problems.
My yellow tang was a rescue, and while he was recuperating in the QT, his various problems were healing fine. This tank has no carbon run in it. After the QT was over, he was added to the main tank where his hlle started coming back. The only difference between the two tanks was the use of carbon.I removed the carbon and the hlle has competely healed up. A few weeks later I was reading the magazine when I came across the small mention of carbon and tangs. Weird huh?
Also make sure that you give your tang some of the green seaweed selects too....