I agree with the previous comments in that you have a perfect storm of bad on that poor fish. This is common in long-term bad husbandry in zebrasoma tangs. Malnutrition and infection lead to HLLE and the fin rot, and velvet is unfortunately common.
I agree with antibiotics and copper treatment, and soaking foods in selcon, zoe, and/or vita chem. Soak strained/drained frozen foods and/or pellets, lots of nori, and unfortunately the fish will probably never be "normal" but you can do a good thing for the fish as you are. Thank you for that!
The trifecta as described is Metroplex, Kanaplex, and Furan 2 for 14 days in QT. Copper can be used in conjunction I recommend Copper Power at 2.0 PPM. You can do 14 days copper if and only if you transfer to a completely sterile QT after 14 days. If not, treat with copper for a full 30 days.