Betta Fish: Does my fish have fin rot? Or what does it have? How can I cure it if something is wrong?

Hi, I’m kind of new to fishkeeping, but I try to give my fish a good life. He has plants, a heater, a filter, and almond leaves, but his fins look like this. What could be happening? How can I fix it?

I’ve had him for about 3 months. He used to live in a very small container, and I moved him to a 35-liter tank. When I bought him, the tip of his fin looked bent. What could have caused that? And what can I do?
Pliss help me 🙁 I love my fish