I bought a heater meant for up to 20g tanks, and I used it for a while in a 10g tank with no problems. It has a temperature dial and shuts off automatically at the right temperature.
Temporarily, my betta is in a smaller, 1.5g tank while I'm living back home, and the temperature is pretty cold. I have a heater meant for small tanks, but I regularly had problems with it creating massive amounts of slime and leading to fin rot.
Would it be unsafe to use the more powerful heater in the small tank? Obviously I don't want to fry my fish, but since it has a thermostat, I'm hoping it would be okay.