That looks like he might have gotten his fin ripped on a piece of decor, a rock, the filter in take, or something like that. If there is something in his tank that has a sharp edge, that could be the cause. Bettas are famous for getting into places that they shouldn't be! The other thing that might have caused it is if he has been flaring. Sometimes when long finned bettas flare, they can make small tears in their fins.
I would do a few small frequent water changes every 3 days for a week or two to keep his water pristine. Clean water goes a very long way in the healing process. Indian almond leaves can also help with the healing process. The tannins that leach from the leaves will tint the water brown, but bettas do love it! If you can't get Indian almond leaves, you could try organic rooibos tea with no caffeine. You can control how dark the tank water gets with water changes.
Wow thank you so much for the reply! Yeah I think it could be from the pink hideout decor I have in there, I’ll take it out and replace it with something smoother and do water changes from here on! I haven’t got around to doing a water change yet, I got him last week how often would you recommend I do water changes? I bought a gravel vacuum today to make it easier
Is your tank cycled? If so, then after his fin heals, I would do 30 to 50% water changes weekly. My experience with bettas is that clean water will prevent a myriad of diseases/health issues, and he will love you for it!!!
If you don't know about the nitrogen cycle, then I am attaching a link for you to read through below. You will need a way to test your water parameters, and also will need a good water conditioner. I don't know if you have access to Prime conditioner by Seachem. If not, there is probably something very similar in your local fish store. The water test kit will let you track the amount of ammonia, nitrite, and nitrates in the tank. The ammonia and nitrites are toxic to fish.
If you know all of this already, please forgive!!!
Perfect, I got a water conditioner, KH powder, bio active for cycling the tank, some chlorine neutral drops and an ammonia thingy to put in the water when I got the tank, I just got back from the fish shop and got a testing kit too and everything seems fine! So I’ve removed the pink hideout decor and I’m going to do a water changes and hope for the best, thank you so so much for your help!