Exactly what Kc said. 99% guess Id say a scratch or battle wound. I am figuring you are meaning the disease called "Dropsy's" from what I have read go get some maracyn-2 at the pet store if it does turn into Dropsy. Otherwise you could add MelaFix for bad scratches or "battle wounds" as I call them. It treats Damaged Fins, Ulcers, and Open Wounds. If its fairly minor just let it heal on its own. Minor scale damage usually heals within 5 days. If its worse than that get some Melafix.