Is it ich? How can I treat my fish?

Hi there! I don’t see any ich. However, it does look like your betta has fin rot. This can be an indication of poor water quality, stress, or both! The first thing to do would be to determine the root of the issue. Are you able to tell us the water parameters (ammonia, nitrite, and nitrate mainly),
the tank size, and if there are any other tank mates living with him? What is the temperature of his tank? What does maintenance look like/consist of? These are some things that will help us determine what is causing it.

If the cause is poor water quality (any amount of ammonia or nitrite, high nitrate) you need to do a 75% water change daily until the parameters stabilize

If the cause is stress, we need to determine what is causing it and remedy it. Stress weakens a fish’s immune system, making way for other issues and diseases to arise.