At the moment I'm not using any heater at all. I let the lights heat up the water a little during the day, then rely on the thermal mass of the water to hold a high enough temperature during the night. When I used a heater, actually a couple of them at different times, they would not set a temperature that I wanted. My tank kept running at 80F or more, with the heater refusing to shut off. I avoided a disaster only because I refuse to use big heaters, preferring to experience somewhat high temperatures with heater thermostat failure rather than making fish stew.