I agree with Wuv, if you are comfortable, chances are the bettas will be too. What I do recommend is getting thermometers for each of the little tanks. I have the little sticky ones that adhere to the outside of the bowl. You are better off being able to see the temperature, and you can adjust if it's too low or high. Not knowing, IMO, is worse. I haven' used heaters in my tank since early May, and my thermostat is set to 75 degrees, so they should be fine this winter and I live in Champaign, IL, where it gets COLD! To help with heat, the tanks are near sources of light, and I maintain the tanks at around 78 degrees.
