Is this a brackish water tank (Mono's, scats, some puffers etc.)??
For freshwater (this includes African Cichlids and catfish) at 1 cup per 15 gallons you have overloaded the tank with salt. There are 48 teaspoons in 1 cup so you are at around 3.2 teaspoons per gallon. This level of salt can be very stressful to many species of freshwater fish and any rise beyond that level could be lethal. Our medicated dose of salt at the store is 2 teaspoons per gallon and for general disease prevention and health we often do only 1 teaspoon per gallon. Regardless of what happens with your catfish, I would slowly reduce the level of salt through water changes to about 2 teaspoons per gallon - and maybe even less after a little adjustment period.
To me, the mark on your catfish looks like a bite mark from a cichlid or another fish with a mouthful of tiny scraping teeth. I would suspect that the salt level, combined with an aggressive tankmate, are the root of your catfishes problems. If you do have a tyrant in the tank and need to get rid of it, you can give us a call and we will probably take him in.
Good luck,
Sam
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