Help! Persistent fin rot

Calling all fish experts! I bought a Betta around November that came with some fin rot (didn't show up for a week) and I've been struggling with it ever since. The first time around I treated him with bettafix (useless junk btw) and when that didn't work I salted the tank and did a decent water change. It stopped the rot and started to heal. Fast forward a few months and I notice it started to come back. Add salt and do a small water change. It seemed to have slowed down the progression but now a couple of weeks later it seems to be getting bad again. I've adequately salted the water, it has a heater and filter. I'm at a loss! Pic of my beautiful boy for attention

I'm going to test the water tomorrow but it's fairly clean!