From the picture I cannot tell much except OUCH! Go to the LFS and check out the medication. Get marycin or marycin 2, see which one will treat both fungel and bacteria and get that one. Hospital tank if this is in a planted tank. There is also the possibilty of it being a fluke, but I cannot tell anything with the picture. If there is no marycin (I think the spelling is close), read what will treat both fungel and bacterial. Might want to take a water sample and get it tested while you are at it.