I am on my first tank, a 1gal. (don't fret, am already planning upgrade!). We keep our house on the cool side. On advice here I set up my tank before I got my betta. I originally had a Tetra 50w from Walmart. It was a dismal failure. The temp never got above 74!
Bought a Hydor 7.5w mini-heater at PetSmart. It keeps my tank at 80-82 during the day, down to about 78 at night. Sometimes at the end of the day I have to unplug it for a bit so it doesn't get too warm when the light is on, but it works great. My betta seems happy & is VERY active! I think he's practicing for the betta decathalon!