Salt baths

[Miss Dib Dabs]

I have a s.gouramie with cotton wool fungus on his tail. I've medicated him plenty for it but it isn't clearing up. I have 100% natural seasalt and wondered how much I would need to use to give him a salt bath in a 1pt jug. Can't add salt to the tank as it has catfish in. Nothing else has it. odd really because the other s.gouramie in a different tank has lately had finrot too!
Please someone give assistance as I'd like to get this under control a.s.a.p.
Hugs,
P.