An aquarium heater is a device used in the fishkeeping

Fishkeeping

Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...

 hobby to warm the temperature of water in aquarium

Aquarium

An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...

s. Most tropical freshwater

Freshwater aquarium

A freshwater aquarium is a receptacle that holds one or more freshwater aquatic organisms for decorative, pet-keeping, or research purposes. Modern aquariums are most often made from transparent glass or acrylic glass. Typical inhabitants include fish, plants, amphibians, and invertebrates, such as...

 and marine aquarium

Marine aquarium

A marine aquarium is an aquarium that keeps marine plants and animals in a contained environment. Marine aquaria are further subdivided by hobbyists into fish only , fish only with live rock , and reef aquaria. Fish only tanks often showcase large or aggressive marine fish species and generally...

s are maintained at temperatures that range from 22-30 °C (71-86 °F). The types include glass immersion heaters and undergravel heating. There are also heating mats that may be placed under the aquarium.

Glass immersion heaters

Most commonly, aquarium heaters are immersion style heaters, these heaters consist of a glass tube containing a heating element

Heating element

A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....

 wound around a ceramic or glass insert. Some glass immersion heaters also contain sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, and most, but not all, are fully submersible

Submersible

A submersible is a small vehicle designed to operate underwater. The term submersible is often used to differentiate from other underwater vehicles known as submarines, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is...

. The glass tube also contains an adjustable thermostat

Thermostat

A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

 which turns the heating element on to maintain the required temperature. This thermostat is often a bimetallic strip; because the strip contains two metals, the metals will expand at different rates when the temperature rises, causing the strip to bend. This strip carries the current, but breaks the current at the correct temperature. The setting of this bimetallic strip can be adjusted. More advanced thermostats may use microchip technology. A small light in the heater is often included to indicate operation.

In a small aquarium, one heater can be used; however, in a larger aquarium, two or more may be used. This has the added advantage of one heater being able to compensate in the event of one heater malfunctioning.

Undergravel heating

Heating of the aquarium can also be achieved via a heating element in the form of a flexible heating cable that is buried beneath the aquarium substrate

Substrate (aquarium)

The substrate of an aquarium refers to the material used on the tank bottom. It can affect water chemistry, filtration, and the well-being of the aquarium's inhabitants, and is also an important part of the aquarium's aesthetic appeal...

. Undergravel cable heating is popular in heavily planted aquariums.

Undertank heating

Aquaria can also be heated by the use of heating mats which are placed beneath aquaria. Glass is, however, a poor conductor of heat and thus reduces the efficiency of this method of heating.