Last update: November 12, 2022

3 Best small aquarium heaters for tiny tanks 1 – 10 gallons (20+ Tested)

One of the biggest complaints with small tanks is their lack of room. By the time you set up your decorations, such as gravel, plants and rocks, there isn’t a lot of room for much else – especially not a heater.

And before you ask, no, you cannot skip buying a heater.

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You see, rapidly rising and falling temperatures cause your fish stress, which can soon lead to death. And, temperatures change much more quickly in a small tank than in a large one.

Besides, tropical fish like betta need warm water to survive.

It is for this reason that a heater is essential to your tiny tank, no matter how cramped it may be.

FishLab tested 20 of the slimmest and most compact aquarium heaters across seven different tanks ranging from 1 to 10 gallons, to answer one question:

Which heater is best for your tiny tank?

Want the answer? Read on!

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The limitations of small aquarium heaters

Small aquarium heater sitting next to a large one

By far, the most common complaint I hear about small aquarium heaters is…

“My heater doesn’t work. The water is still too cold!”

More often than not, the problem is that they are using the wrong-sized heater for their tank.

how to choose the right size of heater

The following three conditions determine the correct-sized heater for you:

  1. The size of your tank,
  2. Your room temperature, and
  3. The temperature you want to raise your aquarium water to.

The power of an aquarium heater is measured in watts (W). The more watts, the more powerful the heater. A 20-watt heater, for example, is twice as powerful as a 10-watt one. The bigger your aquarium, the more watts you need to heat it.

The following table shows the heater wattage you need in order to heat a tank up to 10˚F higher than your room temperature.

1.2 gallons5W
2.5 gallons10W
3.5 gallons15W
4.5 gallons20W
5 gallons25W
10 gallons50W

If your heater cannot warm your tank within 7 hours, it’s probably too small.

Types of small aquarium heaters

First, let’s cover the two different types of heaters you can choose from…

1. Preset aquarium heaters

Preset submersible aquarium heater

Ordinarily, preset heaters have been manufactured to reach a specific temperature – typically 78˚F (26˚C). Once this temperature is reached, the heater automatically shuts off and won’t turn on again until the temperature drops.

However, the majority of the preset heaters designed specifically for small tanks operate a little differently – they are “always on” and never shut off.

Do you have fish that prefer cooler or warmer water? Tough luck! You cannot change it – you are at the mercy of your heater. You don’t have to go far to find horror stories of preset heaters killing fish.

Always-on heaters don’t regulate the temperature. If the temperature of your room increases, say on a hot summer day, then using these heaters will also increase the temperature of your tank – which may be too hot for your fish.

However, you can get around this with the use of an aquarium temperature controller. This nifty device shuts your heater off if your water becomes too hot and turns it back on once the water cools – keeping your fish safe.

The number one advantage of preset heaters is their size. Without the need for a thermostat, manufacturers have been able to create some impossibly slim and small aquarium heaters.

I generally don’t recommend “always-on” preset heaters. But if you have a small fishbowl with less than 3 gallons of water, a compact preset heater will leave you with more space for your critters or decorations.

For the rest of you? The next aquarium heater is the only one you should consider…

2. Adjustable aquarium heaters

Adjustable submersible aquarium heater

These aquarium heaters allow you to select a temperature within a defined range, generally between 66-96˚F (19-36˚C).

Once this temperature is reached, the heater shuts off and won’t start again until the temperature drops below this setting.

While adjustable heaters are often larger than preset heaters, they have the advantage of allowing you to regulate the water temperature according to the fish you keep.

Use the thermostat to make the water hotter in winter and cooler in summer – so your fish don’t cook or freeze.

Now, let’s look closer at what FishLab found to be the best aquarium heaters for small-sized tanks…

Best small heater for 1 to 2.5-gallon tanksFREESEA Aquarium Heater

The FREESEA Aquarium Submersible Heater is one of the best small aquarium heaters you can get today. In spite of its simple design, the product ensures maximum functionality throughout the day.

The FREESEA Aquarium Submersible Heater comes with a simple design that you can submerge in the water. As soon as you turn on the system, it will show you the real-time temperature. Designed for a water tank of 1 to 10 gallons, the system can maintain a temperature range of 59 degrees to 94 degrees Fahrenheit.

You can use a couple of modes to control the internal water temperature. The temperature control unit has to be outside the water, and you can set the desired temperature. Once the internal temperature has gotten to a point, the system will adjust its performance to maintain the temperature.
The entire system, including the temperature control chip, is entirely waterproof with an IPX8 rating. So, you would not have trouble using it in the long term. In addition, because the system comes with a thermostat, you do not have to purchase a third-party thermometer.

You should provide enough time for your fish to get used to the FREESEA Aquarium Submersible Heater. Its heating system attracts fish and may lead to some casualties. However, as long as you watch this aspect, the entire water heater is a superb choice.

● Easy to install
● External control switch
● Constant temperature control
● Smart circuit controls

● It can be a fish trap in some cases
● It cannot be used with big tanks

I used an accurate aquarium thermometer to test and can confirm the heater worked as advertised. Speaking of which, if you need a good aquarium thermometer, FishLab reviewed those here.

Best small aquarium heater for 2.6 to 5-gallon tanks

Best small aquarium heater for two-and-a-half to five-gallon aquariums - Hydor Theo

The Hydor Theo is an adjustable aquarium heater. Most importantly, it’s the smallest adjustable heater on the market.

Want your water hotter or colder to suit your specific fish? Or, maybe you want to turn the heater down during summer. Simply twist the dial on the top of the heater to select a temperature between 67-91˚F (19-33˚C).

Best of all, when tested against our calibrated testing equipment, we found that the heater was within 1˚F of the selected setting.

As you expect, the heater shuts down when the water exceeds the set temperature and switches back on when the temperature drops. An indicator light on the side of the heater lets you know what it’s doing.

Both the 25W and 50W models are 7-inches (18-cm) long, making them perfect for heating smaller tanks, up to 5 gallons.

And yes, the heater is fully submersible. Check it out in a 5-gallon Aqueon Minibow…

Hydor Theo 25W heater inside 5-gallon Aqueon Minibow aquarium

Or, depending on what you have in your tank, you can mount the heater horizontally.

Hydor Theo mounted horizontally in small 5-gallon Aqueon Minibow aquarium

For the most space-efficient installation, I recommend wedging the heater in the corner of your tank like this…

Hydor Theo sitting in corner of a small 5-gallon aquarium

Best small aquarium heater for 6 to 10-gallon tanks

Good news! Once you reach this size, you can use almost any heater you want.

But when it comes to precision heating, size and reliability, there is only one aquarium heater you should pick…Best small aquarium heater for six to ten-gallon tank - Cobalt Neotherm

Out of all the aquarium heaters we reviewed, the Cobalt Neotherm was the clear winner. We don’t just recommend it as the best aquarium heater for small tanks, FishLab recommends it as the best aquarium heater for all tanks!

Accurate to ± 0.5°F, you can be confident that whatever temperature you set is what it will heat to. No other heater we tested offered this degree of precision.

The Cobalt Neotherm is the heater I currently use on most of my tanks. At the time of writing this, the oldest one has been going strong for over 6 years.

See the heater in a 10-gallon aquarium…

Cobalt Neotherm aquarium heater sitting inside small 10-gallon aquarium

Note: The 50W and 75W Cobalt Neotherm models are generally the cheapest.

Want more info? Check out our detailed Cobalt Neotherm Review.

What about a runner-up?

I would recommend the Aqueon Pro as the runner-up, but they are currently in the middle of updating their heaters. The current models are pretty great.

Unfortunately, given Aqueon’s track record with other aquarium products we have tested, such as their air pump, gravel vacuum and water changer, there is no guarantee the replacements will be as good. I will comment once the new models are released, and we have tested them.

Are you looking for an in-line aquarium heater? Check them out.

Even smaller aquarium heaters

This section covers the smallest of the small aquarium heaters.

To reduce the size, these heaters are stripped of virtually every feature. Yep, they are little more than a heating element and a cover.

These heaters are “always-on,” and the only safeguard is you choosing the correct size for your tank.

Let me make it clear that the heaters I listed earlier in the guide are superior in every way.

Due to the lack of safety features found on the following heaters, I don’t recommend them.

The only reason you would choose these heaters is if you value size above everything else. And if that’s the case, you’ll be impressed by just how small these itty-bitty heaters are.

You won’t find anything smaller than these tiny heaters.

First up, we have the Hydor Slim. The smallest aquarium heater for fishbowls…Smallest aquarium heater for fish bowls - Hydor Slim Heater

This circular heater is designed to sit in the flat section at the bottom of your fishbowl.

It’s impressive just how slim this heater is. Cover it with gravel, and you hardly notice it’s there…

Hydor Slim fish bowl heater covered by gravel substrate

Add a fake plant on top to hide the power cord, and you are done.

Fake plant hiding heater in small fishbowl

No other fishbowl heater can be so easily hidden.

Next up, you have the smallest aquarium heater currently available…Aqueon Mini 10W small aquarium heater

At 3 inches long, this heater is impossibly small. Here it is compared to a quarter…

Aqueon Mini 10W aquarium size compared to a quarter dollar coin

And here it is in a 1-gallon tank…

Aqueon mini small aquarium heater inside 1-gallon fish tank

Here is one more option for small square or rectangular tanks…Marina small 8W betta submersible aquarium heater

The Marina Betta is designed for the smallest of tanks, up to 1.5 gallons. And as expected, it is pint-sized…

Marina small 8W betta submersible heater for aquarium size compared to quarter dollar coin

And here it is in a 1-gallon tank:

Marina betta small submersible aquarium heater in 1-gallon fish tank


If you made it this far, then you should have found the perfect small heater for your aquarium.

Just to recap:

The best small heaters for tiny aquariums.

HeaterTank Size
Freesea Aquarium1 – 2.5 gallons
Hydor Theo2.5 – 5 gallons
Cobalt Neotherm5 – 10 gallons

While a tiny aquarium heater is tempting, the smallest lack essential features found in my slightly larger recommendations – these hit the sweet spot between size and safety.

Which heater do you use to warm the water in your small tank? Let me know in the comments below!

Ian Sterling

Ian Sterling, founder of, began his aquarium journey over 30 years ago, driven by a deep fascination for fish and their diverse personalities. His website,, is dedicated to making fishkeeping accessible and enjoyable, offering beginner-friendly guidance, expert insights, and a community for aquarists to connect and share experiences.

Comments (18)

I have every one of your top picks already . Great minds think alike , lol.
I am looking for a new heater for my 80 gallon tank . I see you recommend the 306 controller . Do you have a pick for the heater ?

Hi John,

That is funny! I’m glad to hear you have had the same positive experience!

Honestly, for the heater, it’s either the appropriate sized Cobalt Neotherm, I linked the review in this guide, or the eheim Jager, reviewed here. Some people have a preference for glass heaters and that is still the undisputed king after all these years. I personally dislike it for it’s size, but that’s doesn’t seem to turn many people off. Three of my four tanks currently rock Cobalt Neotherms, the fourth a Eheim jager but this is more because I was impatient and my local fish store was sold out of Neotherms at the time. If you have a good experience with the Cobalt Neotherm and don’t dislike it (the mounting bracket isn’t the best) then I see little reason to try something else.

Unfortunately, all heaters have a chance of failing, even my recommendations. I’d love for someone to make a 100% reliable heater, but let’s face it – it ain’t gonna happen.

I bought the Hydor Theo 25w for my Fluval Spec III (2.6 gallons). But, the temp drops at night are significant enough that the Hydor can’t keep the temp at the set 75 and the water dips to 73.4-ish. In a couple of months, it will be summer and this won’t be an issue, but in a couple of months, maybe I don’t have a fish anymore with nightly temp drops. I don’t have a better place to move the tank that will solve the problem.

Would upping to a 50w work? Two small heaters on opposite sides of the tank (I happen to have a 10w not in use) at night until the weather warms? Suggestions?

I thought getting my kid a betta would be an easy first fish, but things have changed since I was a kid with a fish in a bowl.

Hi Jennifer,

You must live where it’s quite cold. To answer your question, both would work. Either upping to a 50w or adding another 25W heater has the exact same effect. Your 10W may be just enough to keep the temperature up. Alternatively, will turning your Hydor higher temperature at night then down again also work?


Do you have any reviews for heaters for larger tanks, say 75 gallon to 125 gallons? I have read so many reviews that are good and bad with the different brands, it hard to know which to pick.

Hi Natalia,

For larger tanks, I recommend the Cobalt Neotherm (or two, depending on how large your tank is). The Fluval E, despite having an innacurate temperature display is probably the next best pick, but I have had trouble locating it in recent months. The Aqueon Pro USED to be great, when it was made in Italy, it’s now made in China and is a shadow of it’s former self. Please note that ALL heaters have a chance of failure, even one that could be called “the best” Anyone who tells you that there is a 100% safe aquarium heater is lying. Unfortunately, it’s the state of the hobby at the moment.


What size of heater would you reccomend for my 5 1/2 gallon tank? Would one that goes up to 5 be too small?

Hi Marie,

It entirely depends on the temperature outside your tank vs. The temperature you want to raise it to. The bigger the difference, the larger the heater you will need. Those “up to 5 gallons” measurements don’t take your specific needs into account.

If I have a ten gallon tank can I buy a heater for a 20 gallon tank and put it in the 10 gallon. Seems it would work better.

Hi Janice,

It would, but it also costs more and has the problem that if it gets stuck “on” it will cook your fish.

Hello there.
Aqua one nano preheated heater for 10L tank, it’s been in the tank for 11 hours and still the light has not turned off.

Hi Robyn,

Unfortunately, I can’t troubleshoot for you. You will need to contact the manufacturer or return it to your petstore. It looks like that is a UK or Australian brand? I’m not familiar with it.

absolutely avoid the Neo-Therm heaters. They’re good while they last, but unless you have a secondary safety device like an inkbird controlling them, when they go, they’ll nuke your tank. Had the 75w one in my wife’s Fluval Evo 13.5g tank that was good for just over 2 years, then we woke up to a cloudy tank one morning. 95°f was the temp and it was the culprit! Since then, they’ve had NONE available since September for warranty replacement, and have gone cold with any replies! Pretty sad. Presently, I’m running a 100w Tetra Preset heater in my Evo tank and it’s dead on 77.5°-78° accurate. I went with the Cobalt heater even though many reviews mentioned the flaws and I was always talking it up, until September and the loss of about half the reef.

Hi JD,

I’m sorry to hear about your experience. Unfortunately, you’ll find the same thing said about every heater – From Eheim Jagers to Aqueon pros to your Tetra presets. In fact, based on amazon reviews for the tetra preset, there are over 600 one star reviews, many of which blame the heater for failing.

The takeaway: There is no fool proof heater.

The best way to get around this is to buy heaters in pairs (say 2 x 50 instead of a 100) so they can’t nuke your tank or freeze your fish if one fails in addition to a heater controller. Unfortunately, this is the state of the hobby right now.

I understand failures occur, especially with heaters. My issues are more of the lack of support from. the company who issues a 3 year warranty and won’t honor it as they’re out of stock. and offering zero alternatives and have gone silent. The lack. of back end support is quite sad.

Hi JD,

You didn’t touch on that at all in your original comment. Members of my local fishclub have always found Cobalt to be responsive, this is the first I have heard of them being problematic. Thanks for sharing your experience.

I had a Cobalt NeoTherm cook my reef tank last summer. Lost all my fish, corals, and snails. I’ve used Aqueon pros on all my freshwater tanks for years without an issue but I decided to spend more for the Neotherm specifically because it said it had “integrated overheat protection” which apparently it did not. Cobalt did send me a replacement but I’m scared to use it. I’m back to using Aqueon Pro but I did buy an ink bird controller for the reef tank.

When I did water changes in winter, I used the Aqueon Mini 10W to keep the new water at temperature as it sat in a bucket while the pH stabilized (my schedule sometimes kept me from changing the water until the next day). The heater lasted for about two years and just recently stopped working. It was for 25-50% water changes for a 5.5 gallon betta aquarium, so I’ll probably replace it with the less expensive 5W model.

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