I was curious if anybody could help me diagnose and treat this issue with my catfish. I noticed an interesting blotch/rash developing on my catfish's back as well as some listlessness so I quarantined him in a 20 gallon rubbermaid tub to see if i could help him. The original tank is a 70 gallon tall filtered by a Cascade 1000 canister filter and a Penguin biowheel 350, so the water is crystal clear and of great quality (Currently the nitrates are under 20, nitrites at 0, water hardness at 120, buffering capacity is in the 240 range, and the pH is between around 7.6. My last water change (around 20%) was done about 4 days prior to the issue, but this issue was already starting before the water change. I noticed my featherfin catfish of similar size was chasing him occasionally, so he's probably stressed. None of the other fish in the tank show any unhealthy signs. They are all very active and are behaving normally. At first the sore/rash was cloudier. I've put the recommended 1 rounded tablespoon of salt per 5 gallons of water in the quarantine tank and raised the temperature to 82. Now the cloudiness is gone as you can see in the pictures, but the redness hasn't improved yet (he's only been in quarantine for a day though). Any ideas/recommendations?