Hi,
If it is HLLE, it is often nutrition related. What are you feeding your tang? I have had some run in's with HLLE in the past and there are a few things you can do to help. First, make sure you feed your tang a healthy, varied diet with plenty of greens. I use seaweed selects, frozen brine, frozen mysis, and frozen formula 1 for my fish. You should also soak your food in vitamins. Try using Zoecon or Selcon, Zoe, and maybe even garlic. The vitamins will help considerably with this disease. Also, check your water quality, make sure all the levels are where they should be.
Another contributer to HLLE is stray electrical voltage in the tank water from pumps, filters, and heaters. This will contribute to the disease. I purchased a grounding probe to eliminate any stray voltage in my tank, and that combined with vitamins has improved my yellow tangs condition. He is healing up from the HLLE nicely.
Again, the causes of HLLE are poor water quality, not enough vitamins in the diet, and stray voltage. I would start by addressing any of those potential causes and see if it helps. Good luck with your tang, this can be a very frustrating disease, but it is very treatable!
-Steve