I'd just go w/ the 3-in-1 medications if the salt isn't working soon. It could be Fungal, Bacterial or Parasitic.....best to treat w/ something that does all 3 and be on the safe side.My quarantine currently has a couple fish he would probably eat right now. If needed it can wait a bit bc treatment currently means treating 180 gallons. What would be the next course of action. Other thing to note that I posted in the catfish forum is that it's been swimming in circles against the glass for days now. It stops to eat but during the day it's up and down the glass all day.
Try Seachem's Polyguard.....it's has been recommended to me for such instances