gammerus

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.

chickadee

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.

Rose

Butterfly

Maracyne II is a good fungicide. Move it to another tank and treat so you won't have to treat the whole tank and you will use less meds. Sorry your fish is sick
Carol