I've kept bettas in 2.5 and 5g aquariums with success and never used a heater. And actually I keep my home extremely cool, it's rarely above 66 degrees F room temp. And Keller has always been quite active and wiggly in that environment, and confession, I've never even had a water thermometer to check it:) But I've recently moved him to a 10g aquarium and want to guarantee some water temp stability in order to optimize his happiness and longevity. Initially I avoided the heater that came with the aquarium, but I've decided to try it. Got a thermometer, got the heater (which has no temp display) on "low" and his water temp is 78 (definitely warmer than it was yesterday by my touch).
He seems to be thriving, happy and playful, so I'm going to assume the heater did not increase the temp too abruptly for him to acclimate (I did check it in other water prior to adding it to the aquarium).
BUT... I'm a bit paranoid. How fluctuating (or malfunctioning) are heaters? I'm gone quite a bit for work, sometimes days at a time and while I have folks that come feed him I can't rely on them to monitor water temp. Are heaters fairly safe and reliable? Should I turn it off when I leave just in case? I feel like overheating would be worse than chillier water, especially since I've never used a heater and my betta has seemed fine...
Betta: Keller, male VT, not sure how old, I've had 3 months
Cat: Caleb, male black longhaired beautiful rescued mainecoon, 15 years old