I've relied on this page twice for my betta, one to diagnose and two to treat. The page recommends an Aquarium salt remedy and dosage instructions. AQ salt is cheap to, you can get a perfect sized bag for a little over two dollars at walmart. If you want a stronger method, I recommend Jungle Fungus medicine as a good choice but beware, its for tougher cases and can stain aquarium decor. The page is very informative and gives plenty of clear instructions and warnings, if you use the link please follow it carefully. As NickAu said above, a clearer photo will help us to determine if its finrot. Diagnosing correctly is very important before you treat, especially if you have to resort to medicines which are not cheap.I'm not sure what is wrong with my poor guy he has white on the tips of his fins and I don't know if it's fin rot or regrowth! Can someone help!
Make sure you pre soak these for about 15 minutes in a spoon full of tank water before feeding.I feed him hikari dried bloodworms as a treat
How many pellets in total are you giving him? Bettas can be little pigs when it comes to eating.I drop two pellets in at a time til he is done eating