my golden barb seems to have a very odd fleshwound. There is a bloody circle around his anus, and his skin inside of the bloody circle has become swollen and cottony. the tank is clean and he is swimming and eating fine.
It sounds like a fungus infection. He will probably need to be treated with an antifungal medication, usually continuous bath for several days. There are several that are effective. This type of infection is common after an injury of some type. He may have had a small injury that became infected and has now picked up the Cottonwool Fungus.