What Size Heater Is Needed for an Aquarium?

The heater you need depends on aquarium size and ambient temperature.
i aquarium fish 9 image by cherie from Fotolia.com

An aquarium heater, typically rod-shaped, keeps your fish whether tender or hardy in optimal comfort. The size of your aquarium and how much you need to raise its water temperature help you determine how much heater you need.

Heater Basics

Some heaters are marketed for specific aquarium sizes -- 10 gallon, 20 gallon, and so forth. These heaters can provide you with a rough guide, but the sizes are not absolute. For example, a 10-gallon goldfish tank may not require 10-gallon heater, because goldfish are coldwater fish and may only need the temperature raised by a degree or two. More typically, heaters are sold according to wattage. Generally speaking, larger tanks require higher wattage heaters.

Water Temperature

The starting water temperature of your aquarium is a significant factor in choosing a heater. For example, if the temperature in your home is 75 degrees Fahrenheit and your fish need a minimum of 78 degrees, you will require a much smaller heater than someone keeping fish outside or in a very cold room. Take the temperature of your aquarium water without a heater to determine how much you need to raise the temperature.

Choosing a Heater

Most aquarium heaters range from 25 watts to 300 watts. Smaller aquariums require smaller heaters. A 10-gallon tank, for example, will typically need a 25- to 75-watt heater, while a 75-gallon tank will require 250 to 300 watts. Larger tanks may require multiple heaters. If you have a 300-gallon tank, for example, you may require two or more 300-watt heaters. Compare the size of your aquarium with the amount by which you need to raise the temperature. Raising the temperature by 1 to 5 degrees typically requires only very minimal wattage, while raising it by 5 or more degrees requires wattage at the larger end of the recommended size for your tank.

Monitoring Temperature

After you install your heater, monitor the temperature carefully. It can take 24 to 48 hours for the heater to fully elevate the temperature. Increase the wattage you are using if the temperature is still too low and decrease wattage if the temperature is too high. During seasonal changes, you may also need to alter the wattage. For example, if you keep your house very cool during the winter months, you may need to use a higher wattage heater.

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