u/comfortablephonesex

I bought a house recently with a small pond and this is one of 4 koi in it as well as 10 goldfish. Recently this guy developed what I believe is an ulcer. I cleaned the main filter again this weekend and just yesterday a secondary filter fountain with a UV light arrived and I put it in as well. Also did a ~50% water change and I’ve poured in some API Melafix the past couple of days and some pond salt too but it doesn’t seem like it’s healed much at all. How long might it take to resolve and what else can I do to help?