Ok first thing you have to do is GET THAT FISH OUT! I had the same problem with my beutiful 8" Blue Angel. Sadly it did not make it. My biggest problem was I didn't get the fish out soon enough. So, I also lost my rare yes rare Townsend Angelfish because the disease spread to it because I didn't get the Blue out fast enough. So now as I type my Yellow Tangs have it and are just getting better. My Blue Angels life was over in about a week when she first got the disease. I would suggest Maracyn (not Maracyn II) it is working on my Tangs. I would also do a copper treatment on your tank. (I am guessing you don't have a reef tank because you have a Koran Angel.) If you can set on up, I would get a Hospital tank. I hope this helps. You have a good chance to save this fish because you found it early.
Peter
