Do you have any potassium permanganate solution? If not it is available on line. The smallest and cheapest quantity you can buy that I know of is sold under the brand name Jungle Clear Water for about 5 bucks. I have found that for wounds such as this, using a Q-tip dipped in PP to swab the wound pretty much takes care of the problem. If you want to try it, net the fish out of water, cup the fish in your hand head higher then tail--you don't want PP in the gills--and swab the wound. Return fish to clean water.