I have successfully brought back tangs and angels from HLLE. I used vitamin C orally (food supplement), and dosing. It also seems to help when exposing the fish to stronger UV radiation such as halide lights. Varying diet helps as well.
IME lack of nutrition seems to be the main cause, followed by heavy metals and carbon (Is carbon absorbing something the fish need? no one is sure.), then infections (possibly secondary problem).
In any case, none of the fish recovered unless they were moved to a different habitat. Whatever caused the issue was in the system, so keeping them in it was a bad idea. But this is just my experience. It could coincide with some other factor I was missing.