Review on Energy-efficient Betta Heater for Small (1.5 gal.) Tanks - Fully Submersible Aquarium Heater – Automatically Reaches Preset Temperature – Easy Installation with Suction Cups by Mary Ashford

I had trouble finding a heater for my betta's 3.5 gallon aquarium as most of them are designed for large aquariums. Of all the reviews I read, this one had the fewest problems and I needed the heater quickly as my other one stopped working. I've had this for about 2 months and it keeps the tank at a consistent 80 degrees to my standard/glass thermometer (which hopefully is accurate because I've had no luck with digital thermometers). I've noticed that it turns on and off (red light) so I think it controls the temperature of the water. For the time being, I recommend using a smaller aquarium for bettas.