the heater will WARM the water, not cool it.
adjust the dial to where you want the temperature, immerse it in the tank, unplugged, for 30 mins, then plug it in. Any time the water temp drops below what you have set the heater for, it will kick in and warm the water up. It will also shut off once the water is as warm or warmer than the setting you dialed in.
You should also have a thermometer to make sure the heater is doing it's job.