Probably a bacterial infection. Can't really recommend anything to treat it as I haven't had to deal with it personally yet. My guess would be Melafix or Salt to cure it, but I'd wait for some other opinions.
Melafix is great for making your water smell like a tea tree body wash, but I've found it to be useless for any serious problem. If it is indeed a bacterial problem I would suggest a sulfa drug or OXYTETRACYCLINE HYDROCHLORIDE.