Yesterday, I accidentally overfed my Betta fish. Today Is saw the water was murky, so I changed it. Now my fish just stays low at the bottom, and only moves to breath from the top. What did I do wrong?! Can someone please help?

Hi everyone. So I bought a betta 3 weeks ago. I'd had his tank all set up and ready for him to go. It's a 9ltr tank which I think is 2.5 gal. Its heated at 23°C and i use biofilter as the filter i had was too strong for him. Ive been doing water changes weekly. Anyway he has been all happy from...