What is an Inline Aquarium Heater and How Does It Work?

An inline heater looks like a small tube with a heating element inside it. This tube is attached to the outflow hose of an external canister filter. Thus, the purified water, on its way back to the fish tank, passes through the heater and gets warmer. Inline heaters commonly come with a thermostat. Once the water temperature has reached the needed value, it shuts off for a while. Thanks to this, a constant temperature is sustained.

Top-3 Inline Aquarium Heaters: a Comparative Chart

Best Choice
Hydor In-Line External Heater review
Hydor In-Line External Heater
  • Power, Watt: 300 Watt
  • Tank size, gallons: Up to 80
  • Hose, inches: 5/8
  • Dimensions, inches: 4 x 4 x 12.5 inches

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ISTA in-Line External Heater Aquarium Heater review
ISTA in-Line External Heater Aquarium Heater
  • Power, Watt: 150 Watt
  • Tank size, gallons: Up to 50
  • Hose, inches: ?
  • Dimensions, inches: 2.3 x 9 x 2.3

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Aquarium Adjustable External Inline Heater by SunSun review
Aquarium Adjustable External Inline Heater by SunSun
  • Power, Watt: 300 Watt
  • Tank size, gallons: Up to 80
  • Hose, inches: 1/2, 5/8, 3/4
  • Dimensions, inches: 12x 2x 3.5

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Things to Consider When Choosing an Inline Heater

Inline heaters are not made equal. There are some things you need to take into consideration when you choose a device for your fish tank. Check the specifications and read customer reviews before making a choice to make sure this is what you need.

Precise Temperature

An ability to sustain the temperature is a crucial characteristic of any aquarium heater. Check the temp range it supports and make sure you will be able to set up a precise temperature, for example, 78 degrees. Ideally, the fluctuations must not exceed 2 degrees; otherwise, fish will feel uncomfortable.

Efficient Heating Elements

A heating element in an inline heater is hidden inside a plastic casing. It commonly comes in the form of a thin glass tube. A good heating element must raise the water temperature quickly and provide a due level of safety. Ideally, it must have a feature of protection from overheating. If, by accident, a heater stays dry, it must shut off automatically.

Long Equipment Life

The durability of an inline heater depends on its build quality and the materials used. Theoretically, one heater can serve several years if used correctly. Try to opt for the manufacturers that give warranty 1-3 years on their products.

Hydor In-Line External Heater - Original ETH

Hydor In-Line External Heater – Original ETH


Hydor Original ETH is often marketed as the best aquarium heater for external use. It comes with some really nice features, such as an electronic thermostat, providing high precision of temp measuring and a proprietary PTC heater. The latter is reliably protected from overheating, which guarantees the maximum safety of the device. The heater is compatible with 5/8 outflow hoses and can be used for both fresh- and saltwater fish tanks.

What we think

Hydor definitely does its work, and it does it well. It is easy to mount, though if your canister filter tubing has a diameter different from 5/8, you may need to do some DIY work. Keep in mind that the heater is pretty large and you need to place it vertically only. For this reason, the heater can look a sort of cumbersome.


  • easy to install;
  • a precise electronic thermostat;
  • protection from overheating;
  • 2-year warranty;
  • suite for marine water.


  • a bit pricey;

ISTA In-Line External Heater Aquarium Heater


This is a compact and versatile inline aquarium heater with a built-in thermostat. It features a large digital LCD display and a power indicator. ISTA heaters are easy to install; they provide precise temperature control in marine and freshwater aquariums. They are safe to use, thanks to the overheating protection feature.

What we think

The heater seems to be well-built and durable. What is more important, it provides a surprisingly consistent temperature with minimal fluctuations. Thanks to a large LCD display, it is easy to monitor and adjust the current temperature. The only complaint is about the connections supplied. They look cheap, and many customers report on terrible leaks. You’d better replace them or spend some time to adjust them tightly.


  • compact and easy to install;
  • comes with a thermostat;
  • a LED display;
  • high precision of temperature control;
  • for freshwater and marine aquariums.


  • mediocre fittings.
ISTA In-Line External Heater Aquarium Heater

Aquarium Adjustable External Inline Heater by SunSun


This budget-friendly heater is a Chinese product with a surprisingly rich set of features. It comes with three sets of fittings for various types of hoses (5/8, 3/4, 1/2), which makes it a really versatile option. The unit features an LCD display and a power indicator. Thanks to the feature of overheating protection, the device shuts off automatically if the water flow stops for any reason.

What we think

If you are looking for a low-budget inline heater, this option is for you to consider. It is half the price of Hydor Original ETH, while its features are virtually the same. We haven’t found any information about the warranty terms. The fittings seem cheap and the overall build quality is average, but, after all, you get what you pay for.


  • very affordable;
  • three sets of fittings of various sizes;
  • an LCD display;
  • overheating protection.


  • made in China;
  • unclear warranty terms.

How to Best Install Your Inline Heater

Once you’ve bought a new inline filter, you have to install it. The procedure is pretty simple: you don’t need to have any special knowledge or hire a professional; moreover, the manual is commonly supplied. Here are some tips on installation.

  1. Turn off your external canister filter to stop the water flow temporarily.
  2. Attach the tube that is supplied with the heater to its output end.
  3. Attach the outflow hose of your filter to the input end of the heater.
  4. Place the heater vertically (it’s important!) and place the spare end to the fish tank.
  5. Turn on the filter to let the water run through the heater.
  6. Turn on the heater and adjust the temperature.
  7. User a heater controller in order to prevent overheating in case of thermostat outage.

Verdict and Recommendations

An inline aquarium heater is a useful device, which allows sustaining consistent water temperature in a fish tank without cluttering the inner space. In our guide, we’ve reviewed three effective and reliable heater options. Our top pick is ISTA inline heater – a compact unit with a large LCD display and a precise electronic thermostat.






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