Instructions
Calculate the wattage required for your tank. A good rule of thumb is 3 to 5 watts per gallon of water. If you have a 100-gallon tank, you will need heaters with a combined total of 300 to 500 watts.
Decide on the type of heater. Aquarium heaters come in two types: hang on heaters and submersible heaters. Hang on heaters must have the top exposed, while submersible heaters can be fully immersed in the tank. Hang on heaters are less expensive than submersible heaters, but are also less accurate and less efficient. They also don't allow as much choice in where you can place them in the tank.
Mount your heater or heaters near the outflow on the tank's filter. This will circulate the heated water throughout the tank. If your tank is very long, you may want to place one of the heaters at the opposite end of the tank.
Invest in a quality digital thermometer for your tank. Those color-changing, stick-on thermometers are notoriously inaccurate. Floating thermometers are difficult to read. A digital thermometer with a remote probe can be mounted on one of the front corners of the tank, with the probe snaked around the side and into the water.
How to Heat Large Aquariums
Unless you are keeping cool water fish, your aquarium will probably need a heater to keep the inhabitants healthy and happy. It used to be the only type of filter was a glass tube that hung over the side of the tank. Large fish could break the flimsy tubes, and the thermostats were controlled by mercury. More options have become available in recent years. There are submersible heaters, heaters that are buried in the substrate, and heaters built into filter systems.