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.
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!
The limitations of small aquarium heaters
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.
The following three conditions determine the correct-sized heater for you:
- The size of your tank,
- Your room temperature, and
- 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.
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
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
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…
At 4 5/8 x 3/4 x 1 inches (not including the suction cup), this is the smallest preset heater that also includes a temperature control.
The Marina Compact is factory set at 78˚F (26˚C), the ideal temperature for most tropical tanks. Once the temperature exceeds this, the heater shuts itself off until the water drops below 78˚F. The heater then turns on again, and the cycle repeats.
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.
You know what the heater is doing by the indicator light on the front of the unit. It turns on while heating and turns off when waiting for the temperature to drop.
As expected, the Marina compact is fully submersible. You can and should completely cover it with water!
If you need a mini heater, start with this one. The Marina Compact may be slightly larger than other preset heaters, but its ability to maintain a constant temperature is worth the extra length.
Besides, it’s still darn small. Check it out in a 1-gallon Aqueon Minibow…
To say that this is a mini tank is an understatement. But as you see, this compact heater barely takes up any space at all.
It’s also suitable for fishbowls.
We tested it in a 2-gallon bowl and were satisfied with how little space it took up.
Because the suction cup is located at the top of the heater, it can easily cling to the curved surface of a fishbowl.
Have a tiny tank and want a small heater to match? This one is as good as it gets.
Best small aquarium heater for 2.6 to 5-gallon tanks
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…
Or, depending on what you have in your tank, you can mount the heater horizontally.
For the most space-efficient installation, I recommend wedging the heater in the corner of your tank like this…
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.
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…
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.
The smallest 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.
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…
Add a fake plant on top to hide the power cord, and you are done.
No other fishbowl heater can be so easily hidden.
At 3 inches long, this heater is impossibly small. Here it is compared to a quarter…
And here it is in a 1-gallon tank…
The Marina Betta is designed for the smallest of tanks, up to 1.5 gallons. And as expected, it is pint-sized…
And here it is in a 1-gallon 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.
|Marina Compact||1 – 2.5 gallons|
|Hydor Theo||2.5 – 5 gallons|
|Cobalt Neotherm||5 – 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!
I’ve been keeping fish for over 30 years and currently have 4 different aquariums – it’s an addiction. I’m here to teach you everything there is to know about fishkeeping.
I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.