some of my bettas are in heated tanks for spawning but the others are not heated and they live in great condition with a room temperature at 22 degrees at night and 26 during the day, ive had bettas stand at 20 degress before with no apparent effects from the lower end of the scale, as a short measure they should be fine so dont worry a cooler temp will just slow down their metabalism (sp) and this is what keeps them good and dandy. so having the fire on will help keep the temp up but dont put yourself at risk with leaving the fire on whilst you go to bed.