Eheim Jager TruTemp heaters are one of the best-sellers because they are affordable, reliable and they allow you to calibrate the thermostat.

The calibration feature is unique because it allows you to compensate for any differences between the heater itself and your aquarium thermometer. With TruTemp heaters, you get truly precise temperature control which is essential when caring for delicate aquatic life that is sensitive to water conditions.

The heater will be factory calibrated right out of the box. Adjust the blue thermostat dial to your desired aquarium temperature and place the heater inside your aquarium water. Wait 30 minutes before plugging the heater into a power outlet to give the glass time to adjust to your current aquarium water temperature.

Let 24 hours pass. Then check your aquarium thermometer to see whether or not the heater has achieved your desired water temperature. If not it hasn't, you can adjust the red dial to the observed water temperature in your tank. Then reset the blue thermostat dial to the desired temperature. You will have then calibrated the heater to make up for any differences between your aquarium thermometer and the thermostat on the heater.