I just was follow your link about fin rot for your new betta and i am completelly agree whith BETTACHKALOVE advice. For the tank size that you have i would not bother to have the filter. Filters absolutely optional for betta. And that filter really strong and pushing him around that makes him stressed. I would take the filter out and do regular water changes. And don't forget to acclimate your betta:) Let me know if you need any help.
Hi, anhel123. I saw your message from a few days ago. Unfortunately my baby boy Triton passed away several months ago and it took me a while before I could get myself to go out and buy another betta. But only five days after I purchased him, he seems to be losing his fins. :( Terribly rotten luck.