If this fish was in your main tank, with your cories make sure you teat them to even if they don't show signs. Treat main tank, seperate the ill and quartine for 14 days! This strain is both internal and external parasite and should always be looked at and treated like that. Salt baths help ease the pain and draw the fluids out, also a good med will definitly have to be used, especially in your main tank. Usually when cories get this its to late when you notice, I find its best just to treat them if there is a outbreak in the tank it puts peace of mind on hand! Just remember to remove activated carbon when treating