Betta Fish: Can anyone help? My boy has been like this all day, lethargic, hiding a bit or just resting on anything at the bottom of the tank. He’s breathing heavily and his water is good and all tank mates are fine. He ate even though he feels off.

Any advice? What do you think is wrong and what can I do to help my boy? Any and all advice is needed. He is normally very happy and energetic. I’ve had him for a good 4 months and he was already a year old id say when I got him. Does he need a smaller tank? Is he tired? It’s a 10 gallon fully planted filterless sunlit cube tank. He LOVES his home. Let me know what I’m missing/ why this happens.