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Aquarium Heater Size: Complete Aquarium Heater Guide

In the real world, the temperature is not always constant. There isn’t much difference when it comes to fish tanks and aquariums. However, compared to humans and other mammals, fish cannot produce heat nor maintain their own body temperatures.

This means that they are prone to illness due to excessive heat or coldness due to external factors and temperatures from the environment. Hence, having a heater inside the fish tank may help maintain their body temperature levels.

Nevertheless, it is still important to determine the correct aquarium heater size to ensure the correct wattage to avoid overheating or underheating the aquarium.

Types of Aquarium Heaters

Knowing the difference between the diverse ranges of aquarium heaters is an essential step towards the journey of fishkeeping.

There are four commonly used aquarium heaters that can vary depending on their functionality. They are called immersible heaters, submersible heaters, substrate heaters, and in-line heaters. There are also filter heaters and mini-heaters that can be used for specific purposes.

Immersible Heaters

Immersible heaters are placed on one side of the fish tank, usually at the top,  and only its glass-covered heating component is submerged in water. For this reason, they are also known as hanging heaters since they are only partially engulfed by other types of heaters.

Immersible are recommended for beginners, freshwater tanks, and smaller aquariums for they provide adequate heating by dispersing its heating molecules in a compact volume. However, because it is placed almost vertically, it will take a longer time to distribute its heat in larger fish tanks and saltwater aquariums, making them an overall less efficient heater.

Hence, it is best advised to put two hang-on heaters placed on opposite ends of the tank. Moreover, a hole in the hood of the tank is recommended to secure the two heaters in place.

Submersible Heaters

Submersibles are the most common type of aquarium heaters because of their efficiency in heating larger fish tanks. They should only be submerged underwater horizontally or vertically. However, it is best recommended to mount them near the substrate, or the filter, in a horizontal position, to evenly disperse heat throughout the aquarium.

This kind of heater uses a coil-shaped material to distribute heat and is often confined in a glass tube or a hardened plastic cylinder. The main difference is that glasses are prone to breaking if the tank overheats, while hardened plastics are more stable, firm, and less prone to heat damage.

The temperature of the submersible heater can be easily managed because of its LED technology. Hence, if there is a thermometer inside the tank, it can always be visible, due to the light the heater provides. Additionally, some submersibles automatically turn their power off if the water level gets too low, thus, maintaining the temperature throughout the tank.

Substrate Heaters

Substrate heaters, or heating cables, are positioned on the floor of the tank and are mainly used for reptilian aquariums. They usually function as a secondary heater and can be useful for freshwater tanks since they can also eradicate dead spots for their aquatic plants.

The substrate heater’s wires are planted under the tank’s gravel, which results in an even dispersion of heat due to the gentle movement of the water. This allows the roots of aquatic plants to grow inside the aquarium. This feature was so popular in the ’90s, however, as time passed, heating cables are much more expensive now and less used compared to submersibles.

Additionally, it can be a hassle to dig up the gravel just to replace broken heating cables. Thus, substrate heaters are not recommended for sensitive saltwater setups.

In-line Heaters

In-line heaters are made of plastics that are heat-resistant to avoid fires. These are usually placed in between the aquarium’s filter to dissipate heat upwards and back to the tank. This setup may also provide extra protection for the heater since it may help with driving the fish away from the heater.

Most especially, those who keep large aggressive fishes, such as Oscars from the cichlid family, can benefit from in-lines since it prevents the heater from being damaged by the fishes.

However, since the heater is placed near, and almost outside of the fish tank, there are more chances for leakages to occur. This is also the reason why in-lines are more expensive than the other heaters mentioned, thus, this heater is not recommended for beginners.

Filter Heaters & Mini-heaters

Filter heaters, based on the name itself, are filters that also have heating components installed to them. This allows the water to be filtered and heated at the same time. On the other hand, mini-heaters are recommended for fishes in small tanks or bowls that require a certain temperature to be maintained.

For example, Betta fishes can be a challenge for its tanks to obtain its optimal heat measurement, hence, mini-heaters are recommended with this kind of setup. However, it is best to consult your local fish store for the usual temperature of the fish before purchasing any kind of aquarium heater.

Finding the Right Aquarium Heater Size

The right size for an aquarium heater is largely dependent on the volume of the fish tank, the temperature of the tank’s location, and the appropriate temperature needed for the fish tank. A wattage between 2.5 and 5 watts per gallon is the most common knowledge needed to be learned by aspiring fishkeepers.

For example, small aquariums located in warm homes should have a heater size of 5 watts per gallon of water. In that case, an aquarium heater ranging from 50-150 watts is just enough for a 20-gallon aquarium located in a home that is not very cold. It is important to remember that the exact temperature of the room is needed for a specific and measured heater size.

In addition, for tanks over 40 gallons, it is recommended to have two heaters at each end of the water tank. Therefore, placing two 150-watt heaters opposite ends of the tank is more optimal than placing only one 300-watt heater.

An easy calculation is done to know the exact wattage for adjusting the heater size to the room temperature. Subtract the average room temperature from the target aquarium water temperature. Hence, using the chart below, the size of the aquarium heater can be determined. Additionally, simply move to the next column if the heating required is in between the level of wattages.


  • Room Temperature: 20°C
  • Target water temperature: 27°C
  • Heating required: 7°C (27 – 20 = 7)
  • Tank size: 25 Gallons
  • Heater size needed: 75 watts

Guidelines for Aquarium Heater Size

Guidelines for Aquarium Heater Size
Guidelines for Aquarium Heater Size

Additional Tips for Using Aquarium Heaters

  • Placing the heater with high water flow benefits the circulation of heat throughout the tank. This ensures that the warm water produced by the heater will be pushed along the current and into the regions of the cooler water area. However, it is crucial that the heater is placed near the filter’s discharge to prevent removing the good bacteria from the tank due to overheating. Submersibles are ideal for placing heaters horizontally above the gravel so that it is near the filter’s output of water.
  • Submersible heaters are more efficient and advisable than immersible heaters. Although it is often recommended for beginners, immersible that are only partially submerged in water can disrupt the water circulation in the tank and can do more harm than good. However, if immersible is the only option, putting an air stone underneath the heater may help push the cooler water up to the area of warmer water regions.
  • Two heaters are required for larger tanks that are placed in colder temperatures. Depending on the temperature required, for aquariums with bigger sizes, it is best to use two 150-watts aquarium heaters placed on opposite ends, instead of one 300-watt heater positioned at only one side of the tank. Furthermore, for immersible heaters, it is recommended to use more than one unit to avoid straining the heaters and even out the heat dispersion throughout the water. This can also benefit the tank since its temperature will not decrease dramatically if the other units malfunction.
  • The length of the aquarium heater must match the height of the fish tank. Since heat rises, it is crucial that water circulates evenly for efficient aquarium heating. Placing the heater at the filter’s water outlow also adds with dispersing heated water since the water moving flowing out of the filter can travel along with the heated water to regions of cooler area inside the tank. It is also important to check for other heat sources outside of the aquarium since this can have an effect on the overall temperature of the aquarium.
  • Thermometers can be used to measure the exact temperature of the aquarium. It can be hard to keep the temperature uniform through all sides of the tank, hence moving the thermometer in multiple locations checks if the heat is dispersed evenly. Thermostats are sometimes installed in heaters to manually set it to its targeted temperature. Hence, it is crucial to always ensure that the setting is at the correct temperature at all times.
Aquarium with fish
Aquarium with fish


Fishkeeping can be a meticulous and arduous hobby. Nevertheless, it can be rewarding to see your fish swimming happily inside its fish tank.

Ultimately, the type of heater, tank size, room temperature, and target water temperature determines the correct aquarium heater size for your fish tank. Putting a thermometer and moving it around may help to disperse heat evenly throughout the aquarium.

Additionally, placing the heater near the filter’s outflow can also help with maintaining heat equilibrium for the fish. However, even if the conditions are perfect, problems may still arise due to other factors such as heater malfunctioning, or external heat source near the fish tank.

Hence, if there are any related concerns or questions, comment down below.

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