Not enough info. Do you also have a plecostomus? Pleco can munch on unsuspecting sleeping fish at night. Could have run into something rough in the tank or got stuck on ornaments? Remove all dangerous ornaments, if its not baby smooth, remove it. Don't medicate your tank you could irritate the wound. I would go to www.goldfishconnection.com right now and order Medi-Gold (anti-biotic medicated food) to help him recover. I don't think its a disease it sounds like trauma. If he lost too many scales he may die of infection, that's why you need the food. Good news for ya, I've had a GF get stuck behind a rock and rip off about 12 to 15 scales and he recovered just fine. It took a few months for the scales to grow back but he survived and now you can't even tell. Keep your water prestine and for the next few weeks under feed. Keep the nitrates under 10 ppm. If you have a pleco get rid of it, there is no need for one in a GF tank. That green algea is good for the system, let as much grow as you are willing to live with! I wipe mine only off the front glass and it litierally blankets everything in the tank.