When heating your small fish tank, it is important to select a heater that works effectively with smaller volumes of water. Let’s take a closer look at the best heater for small aquariums with reviews of the following: 

Heater Name


More Information

Marina Submersible Heater for Aquarium, Mini, 25-watt

Cobalt Aquatics Neo-Therm Heater, 25 watt

Freesea FS-28 Small Aquarium Heater with LED display

Hygger Submersible Quartz Glass Aquarium Heater Fish Tank Water Heater with Protective Sleeve

Hydor Submersible Glass Aquarium Heater

What Size Of Aquariums Will This Article Focus On?

When we are writing about small freshwater aquariums, what we will be focusing on are tanks that range between 3 to 10-gallons in size. The aquariums in this range are a popular choice for a beginner’s first fish tank. The 10-gallon sized tank is often sold in kit form, with everything a novice will need to get a tank up and running.

Tanks Less Than One Gallon.

Freshwater fish aquariums that are one gallon and below are not really suitable for fish keeping. However if your tank is this size and you would like some options you should look towards the eight-watt Marina Betta Submersible Heater.

Heaters for Tiny Fishbowls.

Fishbowls are a controversial way to house your fish, and we do not recommend them (especially for betta fish - no matter the common myths you may have heard). They offer your pet a poor quality of life and may also cause health issues or even premature death. If you have no other choice than to use such a container to keep your fish, placing something like the four-watt Hydro Slim Heater for Betta Bowls underneath the gravel may be an option.

A Note About Water Temperature And Fluctuations In Smaller Tanks

Fish are unable to maintain their own body heat as they are poikilotherm animals. Because they are unable to generate body heat, it must be generated from the outside. This means that a fish will use the tank environment to help them keep a proper temperature.

Due to this, a freshwater aquarium must be able to maintain temperature ranges between 71 and 86 degrees Fahrenheit on average (and even more specific temperature ranges for certain freshwater fish species). A smaller aquarium will heat up and cool down much faster than a tank that is over 5-gallons.

The heater used must be powerful enough to keep the water at acceptable levels, but not too powerful, as this will cause fast changes in temperature and overheating the environment.

The temperature fluctuations experienced by a small tank requires that any heater used is able to measure temperatures as accurately as possible. They must also be durable enough to withstand the constant cycling that they will go through. Wide temperature fluctuations can cause stress for your fish, and water that is too extreme will cause health problems or even death for your pet.

Types Of Heaters And Their Compatibility With Small Freshwater Aquariums

A heater used for smaller tanks is designed similarly to those used with larger aquariums. A thermostat is set to the desired temperature range and a sensor will monitor the water conditions continually. When needed, a heating element will be activated to help maintain the temperature in the aquarium.

There are several types of aquarium heaters, most of which will not work well with smaller tanks.

Immersible Heaters

These are also referred to as hanging heaters. They may be found in 10-gallon starter kits, but are considered less effective than submersible heaters. With aquariums smaller than 10-gallons, these systems are often too bulky for use.

In-Line Heaters

These types of heaters are located outside of the tank between it and the filtering system. This feature is a negative when space is a consideration (which is often the case for owners with a smaller tank). These heaters also come with a wattage power that is too high for small tanks and would exceed the temperature range quickly.

In-Sump Heaters

This type of heater is also located inside the filter/pump system itself. Size becomes an issue as most filtration systems for a tank under 10-gallons would be too small to house them. Higher wattages are also an issue with these units for the same reasons as with in-line heaters.

Your best option to use will be a submersible heater.

Using A Submersible Heater For Small Aquariums

These heaters are completely submergible within the tank, which eliminates the need for the extra exterior room. A submersible design for smaller tanks must, therefore, be compact in design so they do not take up too much room. Many fish keepers feel that these units are more efficient at maintaining temperatures evenly within the tank.

Another consideration when using these heaters in smaller tanks is wattage levels. There are many submersible products available that offer the lower wattage range required for smaller aquariums. In aquariums that are between 5 and 10-gallons you will want heaters with a level of 50-watts. In aquariums of 2 and 5 gallons you should look towards a 25-watt heater.

Best Small Aquarium Heater Reviews

Marina Submersible Heater 25 Watt

A small heater with a basic design and easy operation.

The Marina Submersible heater is made from impact resistant glass and has a waterproof seal. It uses suction cups for installation and they appear to provide a secure hold on the tank surface.

A bumper guard is provided that adds a layer of security between the heating element and your fish.

The temperature adjustment dial is easy to access and use, but it does not indicate specific temperature degrees which will require you to monitor the water temperature independently.

While this heater is more compact than many others, it is actually seven and a half inches in length, not the six inches advertised.

The lower wattage may prove beneficial to you with a tank that is five gallons or less.


  • Product has waterproof sealing and built with glass that is impact resistant
  • More compact than most standard heaters at seven and a half inches in length
  • A bumper guard helps to protect your fish


  • The 25 watt Fluval will struggle with tanks that are larger than five gallons
  • While adjustable, readings do not indicate temperature to the degree

Cobalt Aquatics Neo-Therm Heater, 25 watt

A heater with a modern look and built to last.

This 25 watt heater offered by Cobalt Aquatics is designed with a shatterproof outer casing that should provide you with a unit that is durable. The modern design is very thin, which is a great feature when used with a smaller tank.

The Neo-Therm is able to supply a temperature range from 66 to 96 degrees Fahrenheit.

A one-touch control system allows you to make adjustments more easily than other heater designs. Thermal protection circuitry adds a nice safety feature that can shut down the heater before it overheats.

You may find this compact unit working overtime with tanks that approach or exceed ten-gallons in water volume.

The LED display offers the temperature setting as well as water temperature readings.


  • The Neo-Therm comes with a three-year warranty
  • A flat design that is one-third of an inch in thickness
  • Can maintain higher temperature ranges up to 96-degrees Fahrenheit


  • his heater comes at a higher price point than some units in the smaller range
  • Water temperature may be one or two degrees less with ten-gallon tanks or larger

Freesea FS-28 Small Aquarium Heater with LED display

An oval design provides a small footprint within the tank.

The FS-28 Aquarium Heater is a small oval-shaped submersible that is more compact than the typical cylindrical shaped heaters. The design incorporates a shield that allows water access to the heating elements while protecting it from damage.

The LED display is very bright, allowing for clear viewing out the readout on the display face.

This feature also may be a distraction for a viewer as they enjoy the fish tank. This product has a memory off function that remembers the temperature settings before a shutdown, which can mean less maintenance after a power outage. 

The readings on the display are in Celsius, which may be challenging if you are not used to this form of measurement.

While the FS-28 has been designed to function with a tank up to 15-gallons, it will prove ideal for an aquarium that is less than 10-gallons in water volume.


  • The oval shape reduces the length of this product considerably
  • Readings are easy to see within the tank


  • The temperature reading for this heater is in Celsius, not Fahrenheit
  • The display is bright and can be distracting when viewing the tank

Hygger Submersible Quartz Glass Aquarium Heater

An averagely sized submersible heater with an additional protective sleeve.

Hygger’s heater is a mid-ranged sized submersible unit. Is built with quartz glass that should prove adequate for light handling. The product comes with a protective sleeve designed to protect the glass as well as your fish.

This heater can be adjusted to temperature settings between 61 and 89 degrees Fahrenheit.

The temperature is adjusted with a turn knob located on the top of the unit and is easy to turn, which is a bonus feature for those who may struggle with turning some controls. The unit may register slightly off with larger volumes of water, but it does provide a consistent level once set. 

The quartz glass element shield may prove brittle if not handled with care.

The fresh and saltwater design should help to prevent corrosion over time.


  • Comes with both an extra sleeve and waterproof design
  • Comes with an adjustable temperature range between 61 and 89 degrees Fahrenheit


  • The heater is 9.8 inches in length, which is longer than some products in this range
  • There may be a slight variance in displayed and actual water temperature in more sizable tanks

Hydor Submersible Glass Aquarium Heater

An incredibly durable submersible heater with easy to use adjustments.

The Hydor glass heater uses PTC design technology which can offer you a glass heater that is resistant to shocks and is more durable than other glass submersible heater designs. The scale on the adjustable controls is graduated in a scale of two degrees, allowing you the ability to adjust temperatures with a minimal amount of effort.

The Hydor will not be damaged if ran dry on accident.

The Hydor is equipped with a cord that is approximately 73-inches in length, which offers more set-up possibilities than some of the other products in this wattage.

This heater would benefit from a reduction in the current length of 10-inches.

This heater offers a stable temperature, supplying the tank with even heating throughout the water column.


  • Will not be damaged during dry runs
  • The materials offer a shatterproof build
  • It is shock resistant


  • The heater is less compact at 10-inches in length
  • The readings may vary by a degree in some situations


After our reviews, we feel the Hydor Submersible Glass Aquarium Heater is the best small aquarium heater. The durable design offers longevity and safety in your tank. The controls are easy to use, and the heat supplied is distributed evenly.

This heater can fit a variety of tanks and tank set-up designs. It provides quality at a reasonable price point. Consider it when you next shop for a heater to use in your small freshwater tank!

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