Since you said your tank is "FO", I am assuming that you have dead or fake corals in there, not live rock??
If so, let me say that I feel that tangs seem to get HLLE in strictly FO tanks. This is probably because FO tanks can not naturally produce some of the algae foods that tangs need to graze on. Essentially, tangs have no grazing opportunities in a fish only tank---thus they are deprived of an essential component to their health and well being. HLLE, as I said, has a lot of variables to it, and can be caused by different things---including stressed immune system, perhaps even a deficient or compromised immune system, lateral line sensitivity to stray voltage that is generated by aquarium electrical equipment, nutritional needs not being adequately addressed, behavioral stimuli need not being adequately addressed [such as grazing opportunity], etc.
First, you need to get a good grounding probe and install it in your aquarium immediately. This will address stray voltage in the aquarium. Tangs are highly sensitive to stray voltage in the aquarium and exhibit this sensitivity by deterioration along the lateral line of the fish.
Get Zoecon vitamin supplement and soak all fish food in it. This is an excellent supplement, containing Omega3 fatty acids, and is specifically designed to help with HLLE.
Address the tangs nutritional needs more broadly, particularly his need for vegetarian algae foods. You can try SeaWeed Selects sold at hobby stores or Nori from the grocery store. You may want to even try making your own fish food mix [much more nutritious than store bought foods].
Try using spinach clipped with a lettuce clip [sold in the hobby] to provide your fish with grazing. Once you have your reef really established you may not need this, but right now, with a FO, this fish should be grazing. The spinach will provide some essential nutrition, but will also give the fish the sense of security it needs by grazing. Note that he may not take to this right away, but keep doing it....likely he will get the hang of it and start ripping into the spinach you provide for his grazing.
Again, do some reading here on other people’s experiences with this problem by doing a Search. Also, I think the fish may well improve if you get the reef setup. I should warn you, though, tangs have been know to eat coral polyps [grazing again].
