Fin rot keeps coming back after salt treatment, help

[florid]

I've been dealing with my betta's moderate fin rot for about a month now. I just finished the salt treatment that Colin recommended - 2 weeks of aquarium salt, 1 week of 10% daily water changes, and another week of 20% daily water changes to get the salt out. The last 20% water change was yesterday. I've been keeping everything clean - cleaning the filter media when needed, wiping the glass, and gravel vacuuming thoroughly. Honestly, I'm having trouble finding any spots in the substrate with a lot of waste. Everything's been kept very clean without crashing my cycle, and my test results look good today. Throughout the treatment, his fins didn't get worse, and I even saw a tiny bit of new growth. But yesterday, I noticed a large split in his lower tail that I don't think was there before. The new fin growth I noticed is gone and frayed now. He still has dark edges on his fin around the tears and splits, which seem more prominent than before. I know this can be a sign of fin rot - he's always had slightly darker edges, but these seem like more than just his color. I'm so discouraged and don't know what to do to help him. He seems happy and active still, but I don't want to feel helpless while his fins just get worse.

Tank stats are still the same - 10 gallons, heated around 79-80 degrees, HOB filtered, and fully cycled. I've got silk plants, 3 moss balls, and some new red root floaters that I just added yesterday. Ammonia and nitrite are still 0, and nitrate is between 10-20. I know it's high, but it's due to my tap water - I dilute it with distilled water to keep it under 20 ppm. No tank mates, just my male betta. He's only been with me since September, so I don't think he's old or anything. I don't see any signs of other health problems.

I've attached some photos of his tail from yesterday - it looks the same today. I've outlined the tears in one of the photos, as they're a bit hard to see, especially the new one behind his anal fin. His other fins look fine, it's just his tail fin that's affected.