Lights are another source of heat for a tank, which is something that many people are not aware of. Lights are necessary for any tank, but there are some heating considerations to keep in mind when purchasing lights. For example, some marine and reef tanks require a lot of light, but slightly cooler temperatures. In these cases, you can get away with smaller heaters and allow the lights to do some of the heating for you. Be sure to keep careful watch over the temperature, though, especially if you cycle the lights on and off regularly.
Chillers are devices designed to cool the water.
A chiller will help to offset this extra heat by cooling the water to the proper temperature. Thermometers are vital for checking the temperature of the tank water. Thermometers range from simple stickers that go on the outside of the tank, to high-tech, submersible or floating thermometers that can monitor water temperature and sound alarms when said temperature goes outside a certain range. There are even some models that can be attached to the heater and chiller and, depending on the temperature, heat or cool the water as needed to maintain a constant temperature. Similarly, some heaters come with built-in thermostats, which turn the heat on if the water gets too cool, and turn it off once the water is warm enough.