I had the same problem with my Koran about 6 months ago.
I never tried anything as drastic as freshwater baths, as I didn't want to stress my fish any more than it already was. Since he was sick when I got him, he was pretty stressed from the move.
Anyway, all I did was try to keep my tank parameters as close to optimal as possible, with regard to temperature, sg, and chemical levels.
Aside from that, the main thing I did was to feed a LOT of nori. Korans love nori, and it helps them get the nutrition they need to maintain a strong immune system.
Anyway, my basic thinking was that if I could make the fish as comfortable as possible, he would recover on his own.
I don't know how far gone your Koran is (mine had about 5 or 6 spots at his worst, along with moderate fin rot), so you may need more drastic measures. It could also be that I just had a strong fish. I'm also sure that my blood shrimp, while not a true cleaner, did help somewhat.
Remember, YMMV but it worked for me.