There are a lot of things that can "cause" sores to appear. Fighting between fish, a scraping up against rocks, and in more serious cases, a cut which got infected, and has grown, or a plague of little beasties attacking your poor guy. If your fish has suffered from this sore for about 9 months now, it's likely you are dealing with a very persistant infection to what could have started out as an innocent nic in the flesh. Usually smaller "new" sores are fairly easy to treat, however, long standing sores, that have become much bigger can pose a much larger problem, and take quite a while to heal. Most of the time, these sore leave a depression on the skins surface (like picking off a piece of a peel from an orange), however, in some cases, the sore can actually be a raised lump, or "bubble" of infected tissue. Either way, it is important you act immediatly, as the wound obviously isn't healing its self, and most likely will continue to get worse. First I would suggest that you clean your aquarium. Now don't go tearing everything apart, and scrubbing with a toothbrush, but do make sure you cover all areas of your aquarium, and be thorough. You might want to buy a scrubbing pad approved for aquarium use, to scrub down the walls of your tank about once a week. Make sure you get all 4 sides. You will also want to scrub down all the ornaments, and equiptment exposed to the water. usually this will not require you to take anything apart or out of the tank unless it is really mucked up. Just scrub it right in your tank. You might want to remove your tubings though, and make sure you clean them inside and out really well. Next you will want to clean your substrate, once again, usually, none of it has to be removed from the tank. If you have gravel, small stones, crushed coral bone etc. as your bottom layer, a manual siphon works great to move it around, and dislodge any debris lying about. If you have sand on the other hand, you will end up sucking more sand out than water, so I wouldnt reccomend that method for sand!!

You can do this 2x's a week, siphoning out about 20% of your water each time. And remember, which ever medication you choose, with about 95% of them you need to remove your charcoal or carbon from your filtration units (if you use those). But keep your filter turned on and running throughout treatment. Without a clear picture, it's hard to tell you exactly what you may be dealing with, as there are several ailments that can cause body sores, and skin ulcers. If you want, you can send me an e-mail with some pictures, and I could better tell you what you might be dealing with. Or you could post them on here. Either way, add some pics., and I will do my best to help, in the mean time, try to make as sterile an invironment as you can, and since we don't know quite what we are dealing with here, it could be contagious, so I would recommend you remove the sore ridden fish immediatly from the others. Keep me posted, and add some pix -k-.
--Emi