If you're new to setting up your own home aquarium, you know there are many supplies you need to check off your list. One of those items is an aquarium heater. Depending on where or how much you have researched, you may have come across the idea that a large aquarium may need more than one heater. However, as with anything, it is a learning experience. Everyone has their own process of assembling a tank and what they think is best for its day to day operations. So while there's not a wrong answer to this question, it is a matter of pros and cons.
About Aquarium Heaters
Aquarium heaters are not complicated devices. In operation, they are set to turn on (start heating) when the water temperature drops to a certain point, then turn off when that temperature is exceeded. They don't really know or care if there is another heater in the aquarium system. All they do is turn on and off when certain temperatures are met.
There are submersible aquarium heaters that can be completely underwater, even down to the bottom of the aquarium, and then there are aquarium heaters that need to be attached to the rim of the aquarium and cannot be fully submerged. These will have a line marked on them that is the minimum water level. Be sure you know which type you purchase.
Advantages vs. Disadvantages
There are some distinct advantages to having more than one heater in an aquarium. If the room that the aquarium is in is quite cold and the aquarium water temperature drops and a single heater can not raise the temperature, a second heater would help boost the heat produced to raise the water temperature. In the case of two heaters in an aquarium, you may want to set the second heater to turn on a degree or two below what the primary heater is set at.
A second reason for having two aquarium heaters is that if one heater fails to turn on (in the case of a burnt-out heater), the second heater will keep the aquarium at the desired temperature. Unfortunately, if the primary heater fails to turn off (stuck in the heating position), the second heater will not help. The only cure for this is to keep an eye on your tank water temperature with the help of a good saltwater aquarium thermometer and remove the faulty heater. Quite often, a faulty heater will leak electricity into the aquarium, which can cause serious problems with the livestock in the tank as well as deliver a shock to anyone who puts a finger or hand into the water (paper cuts on your hand or fingers are a great way to find stray voltage in a tank).
Something else to consider is that if you live in an area that is very cold in the winter, one heater may be stressed to keep up with low-temperature fluctuations during those long winter months. In general, use a heater of 5 Watts per gallon of water, so a 20 gallon aquarium will need 100 Watts of heater power. In large aquariums, it may require two heaters. For example, a 100 gallon aquarium would require 500 Watts of heat, but it would be better to use a 300 Watt heater at each end of the tank, rather than one 500 Watt heater, to keep the water temperature consistent throughout the aquarium.