I once put my tang in QT due to adding an unquarentined spotted mandarin. He infected my whole tank with something. But anyway while the tang was in my 30 gal QT he developed HLLE after about 3 weeks. I put him back into my display shortly after and the HLLE cleared up immediately.
I think the size of my QT and the duration he was in it, caused it to begin in the first place. He had the same diet, same water movement, same water even. But the tank was just to small for him. I didn't give him any meds or garlic or anything like that, it just went away on it's own after he was back in the display.