First step, no more brine. No fish you will keep in SW
will benefit from it unless heavily vitamin fortified or newly hatched naplii. Tangs are herbavors and require constant grazing foods high in spurlina content. Meats are fine occasionally especially with other fish but the tang needs one step further. Usually placing a sheet of Nori in the tank once or twice a day will be good.
If you do have meat eaters as well as the tang I would at the least switch to mysis shrimp and soak in a liq vitamin before feeding. Selcon or Zoecon would be best as they are high in [acronym="Highly Unsaturated Fatty Acid"]HUFA
[/acronym]. Better still making your own >>blender mush
<< will cover all the basis.