Sounds like normal behavior to me. Just because you set the thermostat at 80 doesn't mean that's the temp it'll keep. I've got VisiTherm Deluxes, and when I first plug them in, the light will come on for a minute or so. If the water temp is above the thermostat setting, it will then turn off. I keep my tank between 77 and 78, but last time I looked the heater are set to 82 or so. The exact number isn't correct, but it does keep a consistent temperature.