It's a ten gallon, and the light is off all night and then I turn it on in the morning, so I'm sure that makes the temp go up. I had the same heater in Canada, it only had "higher" "lower" arrows on it. It's off right now, and the temp still goes up to 82 because I turn the light on, so I can't turn it any lower because it's already off. I don't turn the dial every time the heater turns on, just when it's reached the right temp. I don't know what to do, except get a new heater.