That is a lesion that looks like it goes all the way through the fish to the muscle. If it were over the fish's abdomen, it would have died from peritonitis by now. It is a very serious issue. Nobody can tell you for certain how to treat it as there are two (common) possible causes; Uronema (a protozoan) and furunculosis caused by a bacteria (usually Aeromonas). Both require completely different treatments. Is there any chance that the problem started at the site of a previous injury? If so, then it is likely a secondary bacterial infection. If the spot developed all of a sudden, with no apparent injury, I might be thinking more Uronema. A skin scrape looked at under a microscope would rule that in or out. If you can't do that, then it is pretty much a toss of a coin. If the fish can't be isolated for treatment, (I know they are hard to catch!)then I would not do anything at all - it isn't likely to be contagious to the other fish.
Jay
