I have a Betta fish in a 10gal/37.5l heated (78°F/25.5°C) tank. I tested my water and it's perfect I also do regular water changes once a week.
My problem is that all my Betta does is sleep and I don't know if there is something wrong with him.
I have another Betta in a different 10 gal aquarium. And he's fine. The sleepy betta does eat but other than that, he stays at the bottom of the tank.