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The Best Small Aquarium Heater For Your Nano Tank: Wattage & Size Guide Included

Maintaining a consistent temperature in your nano aquarium is key to keeping your fish healthy. So let’s find the best small aquarium heater for your tank!

By Matt Thomas
Last updated on

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best small aquarium heater

If you own a small aquarium it’s particularly important to maintain a consistent temperature, as the smaller volume of water can be more susceptible to fluctuations than in larger tanks. So how do you find the best small aquarium heater for your tank?

We reviewed and assessed some of the best small aquarium heaters available and found the FREESEA 50W Aquarium Heater to be the best for a 5 gallon tank. Or for a 10 gallon aquarium, the Fluval M50 Submersible Heater is a durable and reliable choice.

But we’ve also included heaters suitable for other small aquarium sizes, along with budget and premium options. All so you can find the best small aquarium heater for your needs.

So let’s take a look at the list!

  1. Best For 5 Gallon Tanks: FREESEA 50W Aquarium Heater
  2. Best Budget Pick: Tetra HT10 Submersible Aquarium Heater
  3. Best Premium Pick: Eheim Jager 50W Aquarium Thermostat Heater
  4. Best For 3 Gallon Tanks: Marina Submersible Heater
  5. Best For 10 Gallon Tanks: Fluval M50 Submersible Heater

Best Small Aquarium Heater Picks Reviewed

We assessed the small aquarium heaters below against a range of criteria including wattage versus tank size, safety features, and whether they will physically fit in your tank! So you can concentrate on choosing the best small aquarium heater to suit your needs.

1. FREESEA 50W Aquarium Heater


  • Heater Type: Submersible, adjustable
  • Dimensions: 3.7 (l) x 1.5 (d) x 2.5 (w) inches
  • Wattage: 50 Watt (other Wattage available)
  • Key Features: Temperature range of 59 to 94°F, built-in thermostat, LED temperature display, touch-operated adjustment switch
  • Best For: 5 Gallon Tanks

Freesea’s aquarium heater is a simple but feature-packed unit that’s perfect for small aquariums. Whilst you can use the 50 Watt version in up to a 10 gallon aquarium, we prefer it for a 5 gallon tank. The extra power will maintain your tank’s temperature even in winter.

At only 3.7 x 1.5 x 2.5 inches, it’s one of the smallest heaters available at this size and will easily fit inside your nano aquarium. You can even hide it with plants or decor if you don’t want it to be too visible.

Whilst the heater is small, it provides a wide temperature range from 59 to 94°F. So no matter what species of tropical fish you have, you can set the right temperature given that many species prefer a range of 74 to 80°F.

The temperature is clearly shown on the illuminated LED display. You can even read it from a distance, meaning you can check the temperature easily at any time. And the touch-operated controller is located on the cable, so you don’t have to reach inside your tank to adjust it.

Once the required temperature is reached, the built-in thermostat switches off the heater. So you can be confident your fish won’t overheat. But note this is a submersible heater and does not automatically shut off if the water level drops, so always ensure it’s fully submerged.

Also, be aware some owners report that fish can sometimes get trapped at the back of the heater between the suction cups and the element. So for peace of mind use filter sponge or webbing around the suction cups to prevent this.

2. Tetra HT10 Submersible Aquarium Heater


  • Heater Type: Submersible, preset
  • Dimensions: 5.25 (length) x 1.6 (diameter) inches
  • Wattage: 50 Watt (other wattages available)
  • Key Features: Preset temperature of 78°F, heating indicator light, built-in electronic thermostat
  • Best For: Budget Pick

Tetra’s HT10 submersible aquarium heater is a good budget option for nano aquariums up to 10 gallons. Its compact 5.25-inch length and 1.6-inch diameter will comfortably fit a 3 gallon tank of around 12 inches in height. And you can mount it vertically or horizontally for a better fit.

It’s a preset heater, and while there’s no option to adjust the temperature, it does maintain a consistent 78°F. This is a good general temperature for many species of fish, and if you are new to the hobby can be easier to manage than manually setting the temperature.

The 50 Watt HT10 will maintain the temperature well in most scenarios. But having said that, if you live in a colder climate and are heating a 10 gallon tank the 100 watt version is the better option to ensure it has the power needed.

It has a simple indicator light showing red when the heater is active, and green when the 78°F temperature has been reached. And with the electronic thermostat automatically switching off the heater, it’s one of the easiest to operate.

Some owners have reported concerns regarding the HT10 overheating, and we’d always recommend you keep a separate thermometer in your tank to check the temperature. But many praise this heater for its simple operation and ability to maintain a consistent temperature.

3. Eheim Jager 50W Aquarium Thermostat Heater


  • Heater Type: Submersible, adjustable
  • Dimensions: 9.6 (length) x 1.4 (diameter) inches
  • Wattage: 50 Watt (other wattages available)
  • Key Features: Thermo safety cut-off to protect from running dry, recalibration dial for temperature adjustment, shock-resistant shatterproof glass, heating indicator light, built-in thermostat
  • Best For: Premium Pick

Eheim produces a wide range of Jager aquarium heaters from 25 Watts right up to 300 Watts for large fish tanks. The 50 Watt version is a perfect choice for tanks around 5 to 10 gallons, or you can opt for the 25 Watt heater for a smaller aquarium.

With a 9.6-inch length and 1.4-inch diameter, it’s one of the larger small aquarium heaters but will still fit a 16 (l) x 8 (d) x 10 (h) inch 5 gallon tank if placed at an angle.

It’s fully adjustable, with a wide temperature range from 65 to 93°F, so you can get your water perfectly heated no matter what species of fish you keep. And the temperature dial is large, making it easy to read when you are setting it.

The Jager is particularly good at maintaining a consistent temperature throughout your tank due to the laboratory-grade glass jacket which enlarges the heating surface. This is key in smaller aquariums as they are more affected by temperature fluctuations than larger ones.

There’s a heating indicator so you can clearly see when it’s actually heating the water. And it also has a safety cut-out to automatically switch off the unit if it’s out of the water. So there’s no danger of you accidentally cracking the heater if your level’s too low during a water change.

Note that some owners report the Jager can be a little difficult to calibrate, and there’s also no automatic emergency switch off to stop it overheating when in the water. So do keep a separate thermometer in your tank to be safe.

4. Marina Submersible Heater


  • Heater Type: Submersible, adjustable
  • Dimensions: 6.0 (length) x 1.3 (diameter) inches
  • Wattage: 25 Watt (other wattage available)
  • Key Features: Easy-to-read temperature dial, heating indicator light, fish bumper guard
  • Best For: 3 Gallon Tanks

Our choice of the best heater for a 3 gallon aquarium is the 25 Watt submersible heater from Marina. At just 6 inches in length, it will fit comfortably into a 3 gallon fish tank that’s around 10 inches or so in height.

Although the minimum wattage needed for a 3 gallon aquarium is 15 Watts, this 25 Watt heater gives you the extra power if your climate is particularly cold. And because the heater is adjustable from 68° F to 82° F you can keep it in the 74 to 80°F range for most tropical fish.

It has a heating indicator light so you can see when the unit is actually heating the water versus when it’s on standby. But note that the temperature dial doesn’t have an indicator, just a plus and minus symbol, so you’ll need a thermometer in your tank to get the level correct.

Despite some owners reporting this heater can struggle to reach the high end of the temperature range, by using the higher wattage in a smaller tank you can prevent this. But many owners praise this heater as a compact and inexpensive choice for a small aquarium.

5. Fluval M50 Submersible Heater


  • Heater Type: Submersible, adjustable
  • Dimensions: 11.0 (length) x 1.0 (diameter) inches
  • Wattage: 50 Watt (other wattages available)
  • Key Features: Easy control temperature dial, shock-resistant glass tube, computer-calibrated thermostat, ceramic heat sink for even temperature distribution, heating indicator light 
  • Best For: 10 Gallon Tanks

Fluval’s M series submersible heaters are well-made and durable, and range from 50 Watts up to 300 Watts. The 50 Watt version is perfect for maintaining the temperature of your 10 gallon aquarium, although you could consider the 100 Watt version if you live in a colder climate.

At 11.0 inches in length, it will comfortably fit a standard 12-inch high 10 gallon tank if placed at an angle. It’s also ultra-slim with a 1.0-inch diameter, so while it doesn’t come with a heater guard, you can easily fit a separate one to protect your fish.

The adjustable temperature dial is easy to read and can be set between 66 and 86°F, meaning you can easily choose the right temperature in the range of 74 to 80°F for most species of tropical fish.

Fluval’s M50 contains a ceramic heat sink to help evenly distribute the heat throughout your tank. So again, this will deal with any temperature fluctuations that a smaller 10 gallon aquarium may be subject to.

As this heater is fully submersible do ensure that you position it well below the water line before switching it on, as some owners have reported that incorrect installation can lead to the glass cracking. But overall many praise this heater as easy to use and reliable for a small aquarium.

What To Consider When Choosing The Best Small Aquarium Heater

Maintaining the right temperature in your small aquarium is vital for the health of your fish. Let’s look at the key considerations when choosing the best small aquarium heater for your tank.

aquarium fish with plants in tank

Do You Need a Heater For Your Small Aquarium?

There are one or two species of aquatic animal suitable for nano aquariums, such as cherry shrimp, that do well at the general room temperature of around 72°F [1]. If you own these you’ll be fine without a heater for your small aquarium.

But most nano fish, like larger species, are tropical and need a water temperature in the range of about 74 to 80°F. Choosing the best small aquarium heater for your tank will keep the temperature in this range.

Water Temperature Fluctuations In Smaller Tanks

Keeping your water temperature at a consistent level is arguably the most important consideration when choosing your small aquarium heater.

Similar to nature, smaller volumes of water have a lower thermal capacity and are more affected by outside temperature changes [2]. So your nano aquarium will heat up and cool down quicker than a larger tank. 

A heater will keep the temperature consistent to deal with these fluctuations. This is key, as your fish will become stressed and more susceptible to disease and death if the water temperature is too cold for them [3].

Do Small Aquarium Heaters Need A Thermostat?

It’s much safer to use a heater with a thermostat. The thermostat monitors your water and tells the heater when to switch on or off to keep the temperature at the correct level. It’s the safest way to ensure the temperature doesn’t get too high for your fish.

While the best small aquarium heaters tend to come with a thermostat built-in, there are some heaters that don’t, so check before making your final decision. If you do use a heater without a thermostat, keep a thermometer in your tank and check it daily.

Types of Small Aquarium Heaters

We’ll say up front, that we prefer submersible heaters as one of the most effective and safest types for small aquariums. But we’ve included other types here for completeness.

Submersible Aquarium Heaters

One of the most common and efficient types, submersible heaters are designed to sit fully submerged within your tank. They are usually made up of a coiled metal heating element inside a glass or plastic tube, although you can also get metal ones.

Do make sure you choose one that is compact enough for your aquarium. So in a 5 gallon tank, for example, that may be around 16 (l) x 8 (d) x 10 (h) inches, a submersible heater of around 6 inches in length is ideal.

It’s also critical to always keep these heaters fully within the water when switched on, as they can shatter if they are removed and allowed to run dry without being turned off.

Immersible Heaters

Much less common are immersible heaters, also known as hanging heaters, although you can sometimes find them provided as part of small aquarium starter kits. They hang off the back of your aquarium with the heating element placed inside to heat the water.

These heaters are less effective than submersible heaters and can be difficult to position as the controls need to be above the water line. Having said that, they can provide adequate heating in smaller tanks such as a 3 gallon aquarium.

In-line Aquarium Heaters

We’ve included this type of heater for completeness, as in-line aquarium heaters are actually only appropriate for larger tanks. These heaters are placed on the outflow line of a larger aquarium’s filtration system and heat the water as it returns to the tank.

In-line aquarium heaters are great for larger tanks as they can be hidden from view and heat the water evenly. But often the wattage is too high for smaller aquariums and they are usually for use on a substantial filtration system, such as a canister filter or sump.

Substrate Heaters

Again, substrate heaters are rare nowadays and are generally not designed as a small aquarium heater. They consist of a heating wire that’s fixed to the bottom of your tank and so are not very easy to install when compared to a submersible heater.

They are made for use in planted tanks as they only heat the substrate, rather than the whole tank, to create water circulation within it. This helps feed nutrient-rich water to the plant roots, so promoting growth.

Heater Material

Whilst aquarium heaters used to consist of tubes made of glass, today’s heaters come in a range of materials. From shatterproof glass to tough plastics like acrylic, and even metals such as stainless steel.

Glass and acrylic heaters tend to be tube-shaped, so while there are several compact heaters in these materials, do make sure they will fit your small aquarium.

Metal heaters can come in a range of shapes, such as small discs, that can easily fit in your small aquarium. The material tends to give them a long lifespan, but it also means they can sometimes be more expensive.

How To Choose The Best Small Aquarium Heater For Your Tank

When selecting the right heater for your small aquarium you’ll need to think about a number of factors. Here’s how to choose the best small aquarium heater for your tank.

filling fish tank with water

What Size Heater Will You Need For Your Small Aquarium?

The size and power your small aquarium heater will need is determined not only by the volume of your tank, but also by the temperature of your room, and the temperature you need to maintain your water at.

Many power recommendations are based on a general range of about 74 to 80°F for tropical fish, and an average room temperature of 68 to 72°F [4]. If your room gets particularly cold in winter though, it’s worth choosing a heater with more power. 

This won’t harm your fish, even in a small aquarium, as the thermostat will switch it off once the desired temperature is reached. But by having the more powerful heater it will be able to maintain the temperature your fish need if your room gets colder in winter.

The Appropriate Wattage For Your Small Aquarium

Your heater’s power is measured in Watts (W), and as a general rule, it takes around 2.5 to 5 Watts to heat 1 gallon of water [5]. So taking the high end of this range, a 5 gallon fish tank would need a 25 Watt heater.

The table below shows the wattage needed for various nano aquarium sizes up to a 10 gallon fish tank, to maintain a range of 74 to 80°F based on a room temperature of 72°F.

Nano Aquarium VolumeTo Maintain Approx. 75 to 80°F
3 US Gallons15 Watt
5 US Gallons25 Watt
10 US Gallons50 Watt

But remember do research your fish, and if you have species that require a higher temperature, or if your room gets particularly cold in winter it’s better to opt for a higher wattage than shown here. 

Adjustability And Temperature Control

When it comes to temperature control, small aquarium heaters fall into one of two categories: preset or adjustable heaters.

Preset heaters are designed to maintain a set temperature, usually around 78°F. They are generally cheaper, but you can’t adjust them. So if you have fish that need a warmer or cooler temperature you won’t be able to change it.

With an adjustable heater, you have the flexibility to set the temperature within a defined range – usually around 65 to 95°F. They can sometimes be larger than preset heaters, so do ensure you choose one that will physically fit your small aquarium.

Temperature Readability

The best small aquarium heater should have an easy-to-read numerical temperature gauge, so you can see exactly what you have set it to.

Often this will be in the form of a dial broken into single degrees so you can accurately see what you have set your heater to. In fact, some of the best small aquarium heaters even have digital displays so you can easily see the temperature.

But whichever type you choose, bear in mind that the temperature shown on your heater may not always be accurate to the degree. So always keep an in-tank thermometer in your small aquarium for added accuracy.

Safety Features

Fully submersible heaters are arguably the safest type you can buy. As the whole of the heater sits below the water line there is much less chance of positioning it incorrectly or accidentally running it out of the water, which can cause it to shatter.

On that note, the best small aquarium heater will have an automatic shut-off if the water level is too low or if the temperature gets too high. Also, look for an indicator light that shows when your heater is actually heating the water. Some heaters have an LED display.

Check to see that your heater has a bumper guard that will stop your fish from coming into contact with the heater’s surface or heating element. And finally, if you opt for a glass or plastic heater, make sure it is made from shatterproof material.

Price And Customer Reviews

Whilst heating a small aquarium is unlikely to break the bank, it’s still important to factor in brand and quality when deciding on your purchase. Some of the best small aquarium heaters are likely to be in the $20 to $25 range, but you can find good budget options.

Make sure that you check other customer reviews on Amazon or other online retailers as price is not always the only indicator of quality. Real-world reviews from current owners help you judge whether the product will be the best small aquarium heater for you.

Our Verdict

Maintaining a consistent temperature in a small aquarium is particularly important as they can be much more susceptible to fluctuations than larger tanks.

We found that the best small aquarium heater for a 10 gallon nano aquarium is the Fluval M50 Submersible Heater for its ease of use and even heat distribution. Or if you have a 5 gallon tank, the FREESEA Aquarium Heater is a powerful yet compact option.

Whichever you choose from the list above, remember to check the physical measurements will fit your tank. And if you live in a colder climate we recommend you choose a higher wattage to give the additional power needed.


We’ve answered some of the most frequently asked questions when it comes to choosing the best small aquarium heater. Take a look at the answers below!

What Is The Smallest Aquarium Heater?

Some of the smallest are ‘micro’ heaters for 1 gallon tanks and can be under 3 inches long. However, we don’t include these as a 3 gallon tank is the smallest we recommend.

Can I Use A Large Heater In A Small Aquarium?

Yes in theory, as long as it has a thermostat to switch it off at the right temperature. But it’s much better to get the right physical size and wattage for your small aquarium.

Do I Need An Adjustable Heater With A Thermostat?

We’d always recommend your heater has a thermostat so that it will shut off at the right temperature. An adjustable heater is usually desirable so you can adjust it for your fish. 

Why Do Aquarium Heaters Have To Be Submerged?

Submersible heaters must always be fully submerged so they don’t overheat or shatter. These are the safest type as it’s easier to ensure they are correctly positioned.

Photo of author
Matt Thomas
Matt has been keeping fish since junior high when he used to look after his parent's tank. He loves guppies, cichlids, and his crowntail betta named Bobby.

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