5 Best Small Fish Tank Heater: Small Aquarium Heaters For 20-Gal And Under

These small fish tank heaters are good for tanks with a volume of up to 10 gallons.

It is important to have the best accessories for your fish to ensure they are kept healthy and happy.

Having the incorrect water temperature can cause great stress to your fish, leading to a shorter life span. Or, if tank conditions are not optimal, the wrong water temperature can easily lead to death if not quickly remedied.

Here are our top picks for the best small aquarium heater. Down below we state the criteria we used to determine which is best and what you should look for in a small tank heater.

Smallest Fish Tank Heater

Eheim Jager Nano Tank Heater

  • Top quality adjustable heater with full range of features
  • 25w, 50w, 75w, 100w +
  • Best quality pick
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FREESEA Small Adjustable Heater

  • Adjustable temp 63°F-95°F
  • 25w for 1-3 gal; 50w for 3-5 gal; 100w for 5 - 10 gal
  • Best affordable pick
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Tetra HT Small Aquarium Heater

  • NON-adjustable temperature
  • 50w or 100w maintaining constant temp of 78° F
  • Best budget pick
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I can still remember when my small water heater died on one of my small tanks years ago.

I noticed that the fish were more lethargic than usual, but didn’t look into it right away.

It wasn’t until the next day when I saw one floating that I realized the water was cold. I was living in a hot climate at the time, but I probably would have lost them all if I weren’t!

This round-up contains the most recommended small aquarium heaters that are reliable, effective, and affordable. However, these things can fault for all sorts of reasons, so it’s important to check water temps every day – that’s what I learned from my accident!

Important to remember:

  • Only turn an aquarium heater on once it has been placed into the water
  • Get a covered fish tank, or make a cover for your tank to help trap the heat in – it’s hard to reach and maintain temperature with an open tank
  • If you need to remove a heater from the water, switch it off and let it cool for a while before removing it from the water

Generally speaking, you need 3 to 5 watts of power per gallon of tank water to keep your tank heated to the optimal level.

1. Eheim Jager Small Aquarium Heater

small aquarium heater

No matter what size tank you have, whether it’s a 3-gallon aquarium or a 150-gallon tank, Eheim Jager has a heater for you.

It’s one of the few quality 25-watt heaters available for tanks under 5 gallons, but it’s also available in 50w, 75w, 100w, and so on, all the way up to 300w for a large fish tank.

The temperature is adjustable from 65° to 93° F (18° to 34° C ) making it perfect for keeping the wide range of tropical fish in your home tank.

It has special features that will switch the heater off if it ever ran dry, preventing you from accidentally breaking or damaging the heater when cleaning or doing water changes.

The Eheim Jager aquarium heater has been described by fish keepers as the most reliable heater they’ve ever used.


  • Fully submersable
  • Adjustable temp
  • Shatterproof glass
  • On/Off light
  • Fresh or saltwater tanks
  • Auto switch off
  • Higly accurate
  • Mounting suction cups/brackets

2. Tetra Small Fish Tank Heater


best heater for small fish tanks

The Tetra HT Submersible Aquarium Heater is the best small fish tank heater. 

It can be purchased in either 50 watts (model HT10) or 100 watts (model HT20). Both options are best used for tanks with a volume of between 2 and 10 gallons.

Your choice of model will depend on factors such as the ambient temperature in your house and the size of your tank.

If you live in a cold climate, keep your house rather cool, and have a 10-gallon tank, you should consider getting the 100-watt model. But for most other scenarios the 50-watt heater will be a perfect choice.

This is a small water heater that can be easily concealed behind your tank decorations and plants. The 50-watt model is approximately 5 inches long, while the 100-watt model is just over 8 inches long.

Features of the Tetra heater:

  • This is a fully submersible fish tank heater that is attached with a suction cup to the inside of the tank
  • It can be installed vertically, diagonally, or horizontally – as long as it is underwater it will function
  • Lights to indicate when the heater is reaching temperature (red) and when the water has reached the set temperature (green)
  • Automatically set to heat to 78° F with an in-built electronic thermostat – the temperature cannot be changed
  • Automatic shut-off feature in the event of an electrical short
  • Approximately 5-foot long power cord

3. FREESEA Small Tank Heater

small aquarium heater

This small tank heater is an adjustable heater with a thermometer.

The temperature can be set between 61°F and 95°F. Most aquarium fish need a water temperature of between 76°F and 80°F. Check out some more specific temperatures for different fish down below.

This heater comes in five models, from 25 watts to 300 watts. The 50-watt heater is going to be sufficient for the major majority of fish tanks up to 10 gallons. The 25-watt model is great for 5 gallons and less.

Features of this small fish tank heater:

  • Made of shatterproof glass
  • Double insulated heater
  • Temperature dial on the top of the heater
  • Automatically switched on and off to stay at the correct temperature
  • Can be placed horizontally, vertically, and diagonally in the tank

4. Small Aquarium Heater For Bettas

small tank heater

The VIBIRIT Aquarium Heater is a very small Betta fish tank heater.

This is only an 8-watt heater that is suitable for up to 1.5 gallons of water. This heater is always on and has no thermostat.

It would be a good idea to get a thermometer to go with this small heater. As with most fish, Bettas prefer a tank temperature of 78 degrees.


  • Fully submersible for small tanks
  • Made from durable polymer
  • Automatically reaches set temperature

Choosing The Best Small Heater

The more expensive models are not only made of better quality materials and contain better components.

They also have a range of preferable features. Here are the important things you need to know.

Adjustable Thermostat

Ideally, you want a heater where you can change the temperature.

The alternative is a heater that once on will only reach a certain temperature – usually, 78° F. While that is suitable for a lot of tropical fish, it’s not best for all.

Adjustable heaters will usually have a heat setting range of 65° to 93° F which will capture the ideal temps for all tropical fish. Don’t expect a heater to be able to go much below 65° – you will need an aquarium chiller for that!

You also want a heater that has a high level of temperature accuracy!

Cheap options may have a high margin of error when it comes to reading temperature and switching itself on or off. Something with +/- 0.5° C is perfect.

On/Off Switch

Some very cheap mini-tank heaters are ‘always-on’, but you want one that switches off once it has reached the set temperature.

Even better is when there is an on/off light indicating the unit is functioning well.

Here is one example of an always-on heater, where it is constantly outputting 8 watts of heating power.

The problem is that if the tank temp goes above 78° (on a day when the ambient room temp is quite high – or you have the fire roaring inside in the winter), it will continue to heat the tank and can cook your fish.

Shatterproof Glass

Shockproof and shatterproof glass is ideal when it comes to aquarium heaters.

The last thing you want is for a rock to bang into and crack or damage your heater. Broken glass in a fish tank is a nightmare.

You also want a product that is free from pollutants, resistant to chemical and biological substances, and able to handle temperature fluctuations.

Cheap products may leach unwanted chemicals into your water when heating and cooling.

Other Good Features For A Small  Tank Heater

And lastly, here are some other good features which are nice to have:

  • A safety off switch when the water level drops to prevent it from running dry
  • A long power cord to avoid using extra extension cords
  • Simple recalibration of temperature
  • Able to be positioned vertically or horizontally

And it is always a good idea to have a separate fish tank thermometer from your heater.

This way you can use it to make sure your heater is working accurately and safely for your tank.

Small Fish Tank Temperatures

The standard assertion is to set your tropical fish tank temperature to 78°F.

There are some fish that like a few degrees warmer or cooler, but you should generally set your heater to 78°.

Here are the temperature requirements for some of the most common aquarium fish:

  • Bettas (Siamese Fighters) –  between 78° and 80° F, and never below 74°
  • Tetras – between 72°F – 78°F – will likely become stressed at higher temperatures than 78°
  • Discus – between 82°F – 88°F – should not be kept with fish that need slightly cooler temperatures
  • Cichlids – between 72°F – 82°F
  • Oscars – between 74°F – 81°F – require a constant temperature to remain healthy & well
  • Rainbows – between 80°F – 86°F – as they prefer higher temperatures they go well with Discus fish

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