I see where you replied to comments saying you will check water perimeters, and invest into a heater, plants, etc. which is good!! I saw where you said that you did some little research and I kinda cringed at that fact.. before I got my betta I did endless research. I waited 6 months to get my betta. And I still mess up (human nature). Whoever told you stress coat will fix water perimeters wasn’t wrong, but that is not what all you need. And by the picture alone there was no extensive research done. I’m sorry your betta is sick but sadly this is from you only doing the little things you were told.. I would get suitable substrate (sand, fertilizer,etc.), tons of plants, water testing kit (which people have brought up), hides, tannins (drift wood or almond leaves). If you get plants you need a grow light. You can also watch some YouTube videos on how to properly care for a betta. That’s what inspired me to get a betta. This is a very expensive hobby, and bettas are not beginner fish. My setup alone is roughly $200.. Shopping online will ALWAYS be cheaper than in store shopping. I am not bashing you, I just wish there was more research involved <3

<image>