Aquarium Heater Guide (Aquarium Heater Size Chart)

Maintaining proper temperature in an aquarium is very important for the well-being of your fish. An aquarium heater ensures that the water temperature is maintained at a desirable range. Without a heater, some fish will not be able to survive in an aquarium, especially during the winter seasons. Some fish have very specific temperature requirements, which makes aquarium heaters an important part of the tank setup.

How do I choose a fish tank heater? (Type & Size)

You should choose an aquarium heater based on the heater capacity and the size of your tank. The general rule of thumb is to have a heater capacity of 5 watts per gallon of water.

In addition to the heater size, consider the various different types of aquarium heaters. Aquarium heaters can be broadly categorized into in-tank heaters and in-line heaters.

The majority of aquarists use an in-tank heater that is submersible. They are a great choice because they are accurate, reliable, and cost-effective.

glass aquarium heater

The Best Aquarium Heater

There are many aquarium heaters available today. We have had our own share of multiple heater malfunction, resulting in temperature fluctuations. These are things that you do not want to experience, since it can be stressful for you and dangerous for the health of the fish. In order to prevent these heating issues, our recommendation is to get a reliable heater.

Our top recommendation for most aquariums is the Aqueon Adjustable Pro Aquarium Heater.

This is an in-tank submersible heater with an electronic thermostat. It is accurate to +/- 1 degree. These are common features in submersible heaters, but the main reason why we recommend this product is due to the durable thermal plastic shatterproof housing, along with its auto shut off safety feature. Since they offer it in multiple sizes, it will be suitable for a wide range of aquariums. The 50 watt option will be a good aquarium heater for a small 5-10 gallon tanks. The 300 watt option can support a 90 gallon tank, and multiple units can be installed for larger tanks. Aqueon offers a lifetime warranty for this product.

Aquarium Heater Size

What size heater do I need for my fish tank? First, choose the right heater capacity for the size of your aquarium. Heater capacity is usually measured in watts. The general rule of thumb is to have a capacity of approximately 5 watts per gallon of water. Therefore, a 10 gallon aquarium will need a 50 watt heater. As the tank size increases, the larger water volume is able to retain the heat better. If the tank size is 40 gallons or larger, the watts per gallon can be reduces to approximately 3.5.  

This chart will help you choose the right aquarium heater size:

Aquarium SizeHeater Capacity
5-10 Gallons50 Watts
15-20 Gallons100 Watts
25-40 Gallons150 Watts
50-60 Gallons200 Watts
75-100 Gallons250-300 Watts

Keep in mind that this chart can be used as a general guideline, but the requirements for your specific aquarium may differ. For example, if you live in a very cold area, the heater will need to work harder to bring the water temperature to the target temperature. Therefore, a stronger heater may be required.

Heat retention capabilities will be different for each aquarium. Placing a lid on the aquarium will reduce the amount of heat escaping from the aquarium. Also, acrylic aquariums are known to retain heat better than glass aquariums.

If you have a heater that is larger than 100 gallons, it may be beneficial to have multiple heaters. By placing the heaters in different areas of the tank, you will be able to raise and stabilize the water temperature more efficiently.

In addition, keep in mind that the wattage of the aquarium heater does not always reflect equal amounts of output. For example, a 100 watt aquarium heater made by two different manufacturers may have different aquarium size recommendation. This reflects the difference in the efficiency of the aquarium heaters.

What Size Aquarium Heater Do You Need? #shorts

Types of Aquarium Heaters

There are many different types of aquarium heaters, and you may feel overwhelmed by the many options available. However, they can be broadly categorized into two main types of heaters, in-tank heaters and in-line heaters.

In-Tank Aquarium Heaters

An in-tank heater is placed inside an aquarium. These are the most common type of aquarium heaters. There are three main types of in-tank heaters which are submersible heaters, immersible heaters, and pad heaters.

Submersible Aquarium Heater

Submersible heaters are the most popular type heaters used in aquariums today. As the name indicates, this heater is placed inside the aquarium, fully under water. These heaters are efficient and reliable.

They are often made out of glass material, and they are attached to a suction cup. This allows the heater to be placed on the side of the aquarium wall. The heater can be positioned near an aquarium filter output in order to make sure that the heat is circulated throughout the aquarium.

For efficiency, it can be positioned horizontally near the bottom of the tank. Since warm water rise to the top, this is a good way to make sure that the entire tank is heated. This strategy should be used especially for taller tanks. If you do place the heater near the bottom of the tank, make sure you leave some space between the heater and the gravel layer.

The Aqueon Adjustable Pro Aquarium Heater is our top recommendation for most aquariums due to the durable design and safety feature. This is a good choice because it is an aquarium heater with a protective guard already built in.

However, Aqueon offers a budget friendly option as well. The Aqueon Glass Heater does not have the plastic housing on the heater, but it is made of shatter resistant glass. While less expensive, this model also has a built-in auto shut off function that prevents the heater from overheating.

Immersible Aquarium Heater

Immersible heaters function similarly to submersible heaters, except they can not be fully submerged under water. They are hanged from the edge of the aquarium wall. They are less versatile in terms of their positioning. They also tend to cost less than submersible heaters. While they were more common in the past, they are becoming less common as they are being replaced by fully submersible heaters.

Both submersible heaters and immersible heaters are often manufactured with glass. Therefore, it is important to handle them with care. Invest in a heater made with shatterproof or impact resistant glass. Covering the heater with an aquarium heater guard may be a good idea as well.

Pad Aquarium Heaters

Pad heaters, or mini aquarium heaters, are installed on the bottom of the aquarium floor. These are generally small aquarium heaters designed for smaller aquariums such as betta tanks. Most of these heaters, such as the Hydor Slim Heater have preset temperatures so the temperature can not be adjusted. They are easy to operate, and they can work well for small aquariums and bowls.

In-Line Aquarium Heaters

An in-line heater, such as the Hydor In-Line External Heater, is placed outside of an aquarium. They are connected by a tube line, which can be connected to a filtration system. They can be installed with only a pump, without a filtration system as well.

One advantage of an in-line aquarium heater is the visual appeal. It is not installed inside the aquarium, so it can be placed out of sight. There is no need to try to hide the heater with décor or plants inside the aquarium.

Another advantage of an in-line aquarium heater is the ability to maintain a constant temperature throughout the aquarium. Some in-tank heaters have issues maintaining a constant temperature throughout the aquarium, especially when it is not placed in an area with good water flow. An in-tank heater may heat up one side of an aquarium and shut off, while leaving the other side of the tank under-heated. An in-line aquarium heater will avoid this issue by having the heating mechanism right where the water is being pushed.

Aquarium Heater Placement

Where should an aquarium heater be placed? If you are installing an in-tank heater, it should be placed where there is good water flow. Placing the heater near a filter intake or output would be a good idea. This will allow the heated water to circulate throughout the tank. If the heated water remains stagnant around the area of the heater, the thermostat on the heater may turn off the heating mechanism before the entire tank is heated.

If you have a tall aquarium that has a narrow shape, place the heater near the bottom of the tank since warm water will rise to the top. If the heater is placed near the surface of the tank, the warm water may not have a chance to circulate all the way to the bottom of the tank. Of course, placing the heater at the bottom is only possible with a fully submersible aquarium heater.

Aquarium Heater Controller

How does an aquarium heater work? The thermostat, or the aquarium heater controller, is the component that senses and maintains the water temperature. When the water temperature drops to a certain level, the thermostat will trigger the heating mechanism. When the water temperature reaches a certain level, the thermostat will shut off the heating mechanism.

Small aquarium heaters such as pad heaters and bowl heaters may not have a thermostat. They may also have a thermostat that is present to a certain range of temperature that is applicable for most tropical fish. Keep in mind that their temperature may not be adjustable. In addition, they are typically designed for smaller aquariums, so they will take much longer to heat up a tank. However, they are very simple to operate, and they can work well for smaller tanks.

Mechanical thermostat, or bi-metallic thermostat, is the standard for most aquarium heaters. It is composed of two different strips of metal joined together. It could be composed of two different metals such as brass and iron. In this case, brass would be considered the high expansion metal and iron would be considered the low expansion metal. When the temperature rise, the brass strip will expand at a higher rate than the iron strip, forcing the strips to bend in a particular way. When the temperature fall, the strips will return to its original state. This mechanism will help control the switch in the heater, and allow it to maintain a stable water temperature. They can be controlled by a dial, and can be accurate enough to keep temperatures within +/- 1 degrees.

Digital heaters can be even more accurate. Some models are able to keep temperatures within +/- 0.5 degrees. These thermometers may have external displays and controls that can be placed outside of the tank, which will allow easier temperature adjustments without getting your hands wet each time. However, they can be slightly more expensive than their counterparts.

Aquarium Heater Safety

Are aquarium heaters safe? Generally, aquarium heaters are safe for you and your fish. However, it is important to handle them properly. There are a few safety concerns regarding aquarium heaters.

First, keep in mind that most heaters are made out of glass, and it can break. If a glass heater breaks, it can result in exposed heating elements and broken glass. This can be dangerous for you and your fish. In addition, if the heating component breaks, the water temperature may not be maintained properly.

In order to avoid problems, you should unplug your heater when you are working on a tank, such as when you are performing a water change. During a water change, if the heater becomes exposed and runs dry, it can overheat very quickly. This can break the glass in a short period of time.

The glass heater can break as a result of blunt force as well. If you have fish that are large fish, lots of hard objects as décor, or turtles in your tank, the glass heater may break as a result of an impact.  In order to minimize the risk of your heaters breaking, you can purchase an aquarium heater that is made out of shock resistant and shatter proof glass. Another way to protect the heater against impact is to purchase a protective guard for your aquarium heater.

Second, aquarium heaters can become unsafe for your fish if it malfunctions for any reason. For example, if the thermostat on the heater malfunctions, the heater may no longer be able to maintain the water temperature in the correct range. It may be unable to start heating or stop heating when it is supposed to. This can be dangerous to your fish. Therefore it is important to not solely rely on the temperature display on the heater. Install a thermometer on the opposite end of the heater. Therefore, you will get an accurate reading of the aquarium temperature on both sides of the tank. Be sure to check the thermometer regularly. It is a good habit to check the temperature each time you feed your fish.


Can I use two heaters in my aquarium?

Yes, you can use two or multiple aquarium heaters in an aquarium. In fact, it is recommended to install multiple heaters for large aquariums, especially above 100 gallons or more. There are advantages to using multiple heaters.

First, you can place the heaters on opposite ends of the aquarium. This will make sure that the aquarium is heated throughout the tank, and not just one side of the tank. Of course, good water circulation can help evenly distribute the heat, but having two separate heating points can help heat up the entire tank efficiently.

Next, having multiple heaters will prevent the aquarium temperature from falling drastically if one heater fails. Aquarium heaters are known to fail from time to time, but it is very unlikely that both heaters will fail at the same time.

If you are installing multiple heaters, simply combine the wattage in order to get the appropriate capacity for the tank size.

Can aquarium heaters be fully submerged?

Yes, most aquarium heaters are designed to be fully submersed underwater. However, not all heaters are designed to do so. For example, immersible heaters are designed to be hanged from the edge of the aquarium. These heaters can not be fully submerged underwater. It is important to refer to the manufacturers’ directions regarding installation specifics for each heater.

Do fish tank heaters turn off automatically?

The thermostat in the heater will turn off the heating mechanism in the fish tank automatically. Therefore, as long as the parameters are setup properly, the thermostat will do the job for you.

However, you should manually shut off the heater before you perform a water change. Otherwise, there is a chance that the heater will be running dry. When the heater is removed from the water, it can overheat very rapidly, causing the heater to break. Even if there is a safety feature that prevents overheating, it is always a good idea to manually shut off the heater.

Do aquarium heaters use a lot of electricity?

Aquarium heaters can use a significant amount of electricity compared to other electrical components in the aquarium such as lighting and pump. Of course, this will vary depending on factors such as the size of the aquarium, temperature retention rate, and different between the room temperature and target temperature.

How much electricity can you expect to pay annually for your aquarium heater? Let’s say your electricity is 20 cents per kilowatt hour. If you have a 100 watt heater running 24 hours, the daily usage can be calculated as 2.4 kWh per day. Based on these estimates, you can expect to pay approximately $0.48 a day, or $175.2 a year.

Do all fish tanks need a heater?

No, not all fish tanks require a heater. If you intend to keep cold water fish such as goldfish, koi, or white cloud mountain minnows, you may not need an aquarium heater. This is especially true if you live in an area with mild winters.

However, the majority of fish that are kept in aquariums are tropical fish. Therefore, they must be kept in relatively warmer temperatures. An aquarium heater becomes a necessity especially during the winter when the temperature drops.

Some fishkeepers with many fish tanks prefer to heat up the entire fish room, rather than heating individual tanks separately. In order to heat all tanks simultaneously, all aquariums are placed in an enclosed room, and the temperature of the entire room is controlled with a space heater. This method can be cost effective if you have many fish tanks. The heater does not have to be running all of the time if the fish room is adequately insulated. However, keeping an aquarium heater in the tank may still be a good idea, just in case the heater for the fish room fails for any reason.


Amendments to the COMPETES Act, H.R. 4521 wants to ban many fish and other animals in the U.S. unless they are specifically whitelisted. The House passed H.R. 4521 on the morning of February 4, 2022. The future of H.R. 4521 is now in the hands of the U.S. Senate. Read the latest update from USARK and Reef 2 Rainforest Media.

The website’s advocacy campaign section has a simple online form to send a message to committee members.

Fish Laboratory

With decades of collective fishkeeping experience, we are happy to share the fish care tips that we've picked up along the way. Our goal at Fish Laboratory is to keep publishing accurate content to help fishkeepers keep their fish and aquarium healthy.

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