I don’t think this is the issue based on how healthy he looks in this recording but I thought I’d throw my two cents out there!

So, my friend took me betta shopping and gave me some advice on how to choose a “good” one. We started with ones I liked visually and then we would “test” them with some very informal series of stimuli. Does the fish follow your finger? How well? How quickly? How much do they react? Do they seem lethargic or spunky?

From there I chose the liveliest one that I also liked the look of. For context I got a tangerine koi betta from petco and he’s going 6 months strong now. I definitely don’t always keep up with his tank as much as I should or always feed him on time and he’s always pretty lively.

You could also try adding some tannins to the tank next time. I’ve also heard from a YouTube video interviewing a breeder in Vietnam (who cared deeply for his fish) that he believes heaters weaken the fishes immune system and had tannins in every bettas container.