Please look at the picture of my betta and tell me what is wrong with his fin. He has been treated for fin rot with Maracyn and Ich-x and 50% water changes twice a week for a month. Still eating and behaving normal. He is in a 10 gallon planted tank with snails.
That is looking like fungus to me. I would suggest that you try some methylene blue dips or baths. If you can get it, follow the instructions on the bottle. I would suggest dips or baths, but do not put it into your aquarium. This stuff will turn everything blue, so be careful not to spill it on anything.
I have used it for fin rot, and it helped the fish heal. You might also check out Seachem's stress guard, it you aren't using any thing for his slime coat. It acts like a bandage, and I use it whenever I treat for things like fin rot.