Unlike humans and warm-blooded animals, fish do not produce their own body heat. They must rely on the temperature of the water to maintain their body temperature. Maintaining the proper water temperature is important for the healthy and well being of your fish.

Although some fish do well in cool water, many aquarium fish are most comfortable in water that is above room temperature. Therefore, heaters are an important part of an aquarium environment.

Choosing a Heater
Choosing a heater is largely based on personal preference. Some owners prefer to go with the least expensive type, while others want nothing but the most high tech variety. Still others opt for the most unobtrusive type of heater, so the aquarium looks as natural as possible.

Although there is no single right or wrong heater, there are some differences between them. Before choosing a heater it's wise to learn the pros and cons of the various types of heaters. There are four basic types of aquarium heaters.

Immersible Heater - Often called hang-on, these heaters are the least expensive type. The heater control must remain above the water line, which is the biggest drawback. They can also be more difficult to set at the proper temp. However, the lower price makes this heater very appealing to anyone on a limited budget.

Although I do not use this type for everyday use in my aquariums, they are an inexpensive way to keep backup heater around for emergencies. Whenever I see them on sale I'll purchase a couple to keep on hand for occasions when a heater fails in one of my tanks, and I need a backup heater for a short period of time.

Submersible Heater - These are more expensive than the immersible, but they can be fully submerged in the tank. Setting the temperature is easier, and they can be more readily concealed than the hang off the back type. The big negative is that you pay for what you get, so submersibles are much pricier than the immersible. They are also not easily repairable because the tube is sealed to allow the heater to be completely submerged.

Until recently, all submersible heaters were glass. However, some newer models are now made of unbreakable materials. They price is higher, but it's worth it to avoid an accident. This is particularly true if you have large active fish that are more prone to damaging equipment.

Substrate Heater - Most popular in Europe and among plant enthusiasts, substrate heaters consist of heating cables installed below the substrate. They have the advantage of being aesthetically pleasing, due to the lack of hardware in the tank. They also heat the water more uniformly than immersible or submersible types.

Substrate heaters also promote plant growth, which makes them popular among those who keep aquariums with live plants. Unfortunately the downside is that the tank must be torn down to install or service them, and they may be difficult to find locally.

Filter Heater - This is the newest kid on the block when it comes to aquarium heaters. Consisting of a heating block that is made to fit in the filter, it heats the water as it passes through. The benefit is that no equipment is placed inside the tank, thus making this type athletically appealing. Another benefit of not being placed inside the tank is that they cannot be broken by boisterous fish.

A variation of the filter heater is the in-line heater. These heaters are designed for larger aquariums or aquarium rooms, which utilize drilled tanks with pipes and a holding sump to circulate water to and from the tank(s). The in-line heater is placed in the line between the sump and the tank, and heats the water as it passes through.

Next Steps
Once you've chosen your heater type, you should calculate the wattage needed to keep your tank at the optimum temperature. The heater size guide will help you come up with exactly the right size heater for your needs. If you are debating between two sizes, err on the side of a larger heater.

Keep in mind that heaters can, and will, fail from time to time. Nothing lasts forever. Always keep a spare on hand for emergencies. The spare doesn't have to be expensive, or even a large one, just something to provide heat until you can get a permanent replacement heater. Lastly, if you aren't sure where to position your heater, read the heater placement article for tips.