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Aquarium Heaters

What Kinds of Aquarium Heaters Are There and What Is My Fish Tank Heater For?

(The First Tank Guide)

An aquarium heater is a piece of aquarium equipment used to keep the fish tank water warm - or, rather, to keep the fish tank from getting below a particular temperature.

A heater will be necessary for your fish tank if the temperature in the room the fish are being kept in is regularly below the preferred temperature for the fish, or if the temperature in the room ever drops below the minimum temperature that is safe for the fish. In general, an aquarium will require a heater for tropical fish, but not for temperate fish.

Most aquariums have heaters, but there are often questions about these very common pieces of fish tank equipment. There are several types of aquarium heaters available, both internal (the heating element is in the fish tank) and external (the heating element is not in the fish tank). External heaters come in two varieties: in-line heaters and in-filter heaters, though there occasionally mention of in-sump heaters (don't worry, I'll cover those too). Internal heaters come in three varieties: hanging, submersible, and substrate. All heaters also need a thermostat to switch the heater on and off as necessary to maintain the desired temperature.

Hanging Aquarium Heaters

Hanging heaters are the most common and least expensive heaters available. Almost all aquarium kits contain a hanging heater to help new aquariists maintain correct water temperature for freshwater tropical fish.

These heaters hang (as the name implies) off the top edge of the fish tank, usually at the back of the aquarium, with a glass tube containing the heater element set into the water. A hanging heater will require you to cut an opening in the aquarium hood to accommodate the head of the heater, but most aquarium hoods come with a section designed to be cut to accommodate hanging heaters or filters.

It is also important to make sure you secure the heater to the back of the aquarium correctly so that it does not get knocked about, as this could break the glass sheath on the heater, posing a risk of electrocution to you and your fish, and possibly a risk of fire.

It is important to note that hanging heaters are inappropriate - even dangerous - for marine or brackish water aquariums, as the salt can get into the tube and cause corrosion or electrical shorts.

Submersible Aquarium Heaters

Submersible aquarium heaters can be fully immersed in the aquarium water. Because these heaters are completely submersible, they are generally more efficient than the hanging aquarium heaters. These heaters can be placed fairly low in the water. submersible aquarium heaters can be positioned vertically, horizontally, or at an angle, however, they tend to work best when positioned vertically or horizontally. If your submersible heater has an internal thermostat, it is usually more efficient if you position it horizontally in the tank, and fairly low in the water column. This will help the thermostat get an accurate reading on the tank temperature and correctly activate and deactivate the heater.

Submersible heaters are usually attached to the back of the aquarium by a clip with suction cups. It is important that you keep the heater off of the gravel, as the difference in heat conductivity between the water and the gravel could result in the glass of the heater cracking. It is also important that you provide enough space between the heater and the gravel or the side of the tank to insure that no fish will get caught against the heater and get burned.

Aquarium Substrate Heaters

Aquarium substrate heaters are the least common of the internal aquarium heaters for hobby or pet keeping use. Substrate heaters consist of a coil or grid of wire in an insulator which is buried in the aquarium gravel or substrate. The wire itself is the element for the heater, and when the heater is on, this wire becomes warm and heats the gravel in the tank. The heat is then radiated from the gravel to warm the water. When the water becomes warm enough, the thermostat will switch the heater off.

Substrate heaters are supposed to be particularly beneficial for planted tanks where the gravel may act as an insulator and keep the plants' roots too cool. With a substrate heater, this concern is alleviated because the heater keeps the gravel nice and warm, and keeps the plant roots happy and comfortable.

In-Line Aquarium Heaters

In-line aquarium heaters are external aquarium heaters which are self-contained, but reside in or along a section of external plumbing. This is usually the plumbing for some kind of aquarium filter or other piece of external aquarium equipment.

In-line aquarium heaters require a water pump to move the water through them, though this can usually be piggy backed with a trickle filter, a canister filter, or a UV sterilizer, and all the heater requires is water flow. The water is pumped out of the tank, through a tube or pipe, through the heater, and the heated water is returned to the tank.

In-Filter Aquarium Heaters

Some models of aquarium filters, usually canister filters, but sometimes power filters or other varieties, come with built-in heater elements. These heaters heat the water as it goes through the filter, returning heated and cleaned water to the aquarium. Since these filters are inside of - and usually integral to - the filter, they are referred to as in-filter heaters.

In-Sump Aquarium Heaters

Occasionally you may hear about in-sump aquarium heaters or sump heaters.

Usually an in-sump aquarium heater is nothing more than a submersible aquarium heater that is set up in the sump of a trickle filter rather than in the fish tank itself. This configuration provides better safety for the fish by minimizing the risk of the aquarium heater getting damaged by a belligerent fish such as an Oscar, making it much less likely that something would get knocked against or fall onto the aquarium heater and breaking it, making is less likely that the aquarium heater would get damages while the aquarium was being cleaned, and by making it impossible for a fish to get caught under or behind the aquarium heater and getting burned.

Other Considerations

It is important to make sure that your aquarium heater is set up so that it is unlikely that a fish will get caught between the heater and something else, such as a decoration, the side of the aquarium, or the aquarium gravel. It is also important to make sure that there are plenty of other hiding places in the tank through proper aquarium decoration so that the fish do not feel that the only hiding place is against or behind the heater. A fish that gets stuck between the aquarium heater and something else is definitely at risk getting severely burned, and even fish that feel there is no place else to hide can receive life-threatening burns from your aquarium heater.

Make sure that there is water between the aquarium heater and any other surface, especially around the element inside the heater. If the glass sheath of the aquarium heater touches the aquarium glass, gravel, or a decoration, this could cause a temperature gradient in the glass which will cause the glass to shatter. This exposes you and your fish to electrocution, and can pose as a fire hazard.

When you first set up your fish tank, or when you get a new heater for your aquarium, remember to always let the new aquarium heater sit in the aquarium water, with the heater properly set up, for at least half an hour before you plug the heater in or begin to set the thermostat. This will give the heater time to reach temperature equilibrium and prevent the glass from breaking due to temperature differences when the heater element comes on. Similarly, it is important to make sure that the heater has been unplugged for half an hour or more before removing it from the fish tank to insure that the glass is cool and unlikely to break on contact with the air (or some other surface) or due to water evaporating off the glass, and to prevent the heater from causing damage by burning or melting anything it may be set on.

What If I Do Not Want a Heater in My Aquarium?

An aquarium heater is a very important part of your fish tank set up if you are going to have tropical fish. Many beginners forgo the heater and instead select temperate water fish. There are a variety of goldfish and other temperate fish which will do quite nicely at room temperature or sometimes even cooler.

"Thank you so much! Very helpful! Of course, there's more info here than we need, but it more than adequately answers my question and will certainly help both now as we set things up and in the future as we "grow" from here potentially."
January 20, 2004
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"[...] you seem like the "go-to" person for any [aquarium] questions we have! Thanks for your time!"
January 15, 2015
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