I am so pleased to hear the 'plant filtration' idea worked for your fish, trennamw ! I also find the nitrates tend to get lower.. frequently they test at zero.
I'm finding that since the heating system here was changed.. [ wretched landlord, grrrrr ].. I'm going to have to find a way to give my Bettas in the bedroom some heat. So long as it didn't drop below 76 they were ok, but now they're getting too cold at night, poor guys.
I think I'm going to divide a 10G for 3 of the Bettas. I'll run it just like I have the 2.5s, but they can share one heater. I think I have come up with a good way to divide the tank so they can't see each other and won't be able to bypass the dividers either. Neodymium magnets, siliconed into vertical holders with plastic canvas mesh fitted in between. Another magnet will hold them from the outside of the glass.. no suction cup to wear out. Guess I'll find out if the idea works or not.