Catfish are scaleless and he may be showing signs of burning from the meds you have used. Scaleless fish are very sensitive to meds and you can burn them if you are not careful. I would give him a dose of stress coat to sooth the skin and just let him be in some clean water and see if that does not heal the welts.

you might even consider a temporary hospital tank if the red tail is still young and small. that way, the gravel isn't there to hold too much decomposing matter. if you do, give it something to hide in. find more advice on hosp, tanks than i give here, though.
anyone think it would be too stressful to be moved?

How old is the tank and have you tested the water ? I'd start with some water changes, small amounts, often. I agree with the burning from the meds. You might want to look for some medicated food to. There are a number of them available for Koi and Goldfish and I would assume these would be fine for a Catfish.