Betta Fish: I need some help with fin rot please!!

I’ve seen different posts with different treatments like simply keeping the water clean, adding tannins, getting medication like Kanaplex, doing salt baths. I’m just wondering what really is the best way to do this.

I did a water change around 2 weeks ago and saw his fins were looking a bit curled and weird at the ends. Since I saw keeping clean water can help, I made sure everything was good. Last night he suddenly got 5 times worse. I don’t even know how over a few hours his fins could disappear to that extent. I did another big water change today and tested parameters and other than maybe a little too many nitrates, I didn’t see anything wrong. I really want him to get better and I feel so bad I let him get worse.