Temperature Control is easy in a small, simple tank such as ours, but it can be more involved in a larger reef tank.  In our tank, a Hagen Tronic 200 watt heater, located in the sump, maintains a constant temperater between 80° and 82°F.   However, factors such as submersible pumps, filters, or lighting can cause the water temperature to fluctuate.  My heater is right next to the pump in our sump.   I'm sure that the pump creates heat as its working, and I've noticed that the heater rarely comes on.  The lighting hood also generates a fair amount of heat during the daylight hours.  So, the combination of these pieces of equipment work together to keep a constant temperature in the tank.

In larger reef tanks, where MH (Metal Halide) or other intense lighting is present, the lighting can generate too much heat, causing the temperature to go higher than desired.   A series of cooling fans on the lights and/or a Chiller is needed to "chill" the water back down to the desired temperature.  Chillers can be anything from an old college dorm refrigerator (modified to allow tubing to wind through it, of course) to a high tech (and high buck) commercial model.  I do not have, nor need a chiller at the present time.   However, when we move... I'm afraid we may need one for our larger tank.

Below is a simple table that shows what size heater is needed for your aquarium.   Just find the tank size in gallons or liters, and match it with the correct wattage.  Remember the extra heat factors - pumps, filters and lighting!