You would usually need one aquarium heater for your tank. And depending on the size of your tank, you’ll need to choose a tank heater with the appropriate wattage levels. You can find a guide with the proper wattage-to-tank-size ratio in the previous section. If you have a significantly large aquarium, then consider buying two heaters. Instead of placing them beside the other, make sure they are installed on opposite sides of the tank. That way, heat will emit from both sides equally.