If you are setting up an aquarium for your home, you will most likely need a heater as part of your basic equipment. Most aquarium fish are tropical, meaning that you will need to heat the water in your aquarium to keep it above average room temperature. Even if you live in a warm climate, a heater is needed to maintain a constant temperature in your tank. Constantly fluctuating temperatures can be damaging to your fish and plant life.

The required water temperature of your aquarium will depend on what kind of fish and plant life you stock it with; a large reef tank with tropical fish will require higher temperatures than an aquarium approximating a river ecosystem. Consult with your fish dealer. Once you know the required temperature, make sure that you purchase a heater with sufficient wattage to maintain that temperature. If your average room temperature is 68 degrees Fahrenheit and you need to heat your water to 78 degrees, then you will need to raise the temperature by 10 degrees. As a simple rule of thumb, to raise the temperature by 10 degrees, you need 5 watts of heating power for each gallon of water. So, if you have a 100-gallon tank, you will need 500 watts of heating power. There are various tables both online and at aquarium stores that can help you calculate the wattage that you will need for your tank.

Heating units are available in a broad range of wattages; if you have a larger tank (say, 60 gallons or more), it’s often a good idea to get two heaters adding up to the total wattage required, and to place them at opposite ends of the tank. This will provide a more equal distribution of heating power, and ensure that your entire tank is consistently heated.

The most basic kind of heater is an immersion heater, which hangs on the back of your tank; it is fully submersed (with thermostatic controls at the top of the unit, above the water line), and consists of glass or stainless steel tubes containing a heating element that is wound around a glass or ceramic insert. These units must be submerged in water when in use; if they are left on while outside the water, they will overheat and burn out. Most have a built-in “safety” or automatic shut-off switch that turns the unit off if it is not submerged. Immersion heaters require little maintenance; a mineral plaque may build up over time, but this can easily be removed with steel wool.

Titanium immersion heaters are more durable than regular immersion heaters, but they are also more expensive. The heating element is virtually indestructible and will not shatter if bumped. Also, the outer casing is made of metal, not glass, so it too is more resistant to bumping. The thermostat unit in a titanium heater should be built into the unit, as with regular immersion heaters; some titanium immersion units have separate thermostats, but most aquarium enthusiasts find this inconvenient.

Another type of heating unit is an undergravel cable heater, which is a heating element coated in thick flexible rubber designed to be buried beneath your aquarium’s substrate. This kind of heating unit is effective if you have live plants; heating the substrate creates a gentle flow of water through the gravel, enabling your plants to absorb more nutrients from the circulating water.

If you have a smaller aquarium, you might consider a heating mat, which rests underneath your aquarium. The mats are made of synthetic material concealing a heating element. They are not suitable for larger aquariums but may be used for a series of smaller aquariums that don’t require much heating power, or that are too small for an immersion heater. Such undertank heaters are generally used for terrariums housing reptiles and amphibians. A substrate must always be used, and the heater must be controlled by a thermostat, to prevent overheating. Check the heater regularly for discoloration or wear and tear; malfunction could result in overheating or even a fire.

A relatively new alternative is an aquarium filter heater: a heating unit that rests inside the aquarium filter unit, usually a canister filter, and heats the water as it passes through the filter canister. These are the most aesthetic choice, since they reside outside the aquarium and are completely hidden by the filtration unit. Various models of filter heaters are designed to work with specific canister filters; consult with your dealer.

An aquarium heater is only one component among many when you are first assembling your aquarium project, but you must be sure to purchase a heater that is appropriate for your tank size and conditions.