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.

How long should a inline water heater run?

Inline aquarium heaters are integrated into the filtration system. They are a long-lasting type of aquarium equipment. Normally, a high-quality inline heater can run all the time for several years. You don’t need to turn it off – just set the desired temperature and check it once a day with an independent thermometer. It’s important to buy an inline aquarium heater from a reliable manufacturer, which provides a warranty of at least one year for its products.

How do you build an inline heater?

Today there is a myriad of effective inline heaters at different prices for aquariums. Finding a perfect one for your tank won’t be a problem. However, if you’ve made up your mind to build one yourself, here are a list of the parts needed and an assembly guide for you: 38mm elbow with one side threaded; 38mm tee with one opening threaded; threaded reducers from 38mm to 12.7mm; a smooth reducer; 38mm PVC pipe; 12.7mm nylon hose barbs adapter; Teflon tape and PVC cement for joining the parts. To assemble the Elbow and Tee pieces, attach the threaded reducers to them. Afterward, put in the hose adapter. Now pass to assembling the housing but don’t cement them, yet. Use silicone glue to attach the thermometer to the smooth reducer. You can use a cord grip, as well.

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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Also great
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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Also great
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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Gilbert Jones

Fish tank water heater is probably the most important equipment in your aquarium. Water heaters which are reliable, safe for your aquarium creatures. Also high-quality can create a comfortable environment for your fish tank inhabitants in any season. It is highly important to keep certain aquarium temperature because some fishes and plants species needs higher or lower aquarium water temperature than the regular room temperature. It does not matter if you have large or small fish tank. You still need the best aquarium heater to ensure your fish tank inhabitants well-being.

Naomi Obrien

Hydor In-Line External Aquarium Heater – While submersible aquarium heaters are fine for small to medium-sized aquariums, they are less efficient for larger tanks. The Hydor In-Line External Aquarium Heater is an in-line heater that hooks up to your filter line to heat the water before it enters your tank. This heater is suitable for both freshwater and saltwater applications and it is very easy to use. The Hydor In-Line External Aquarium Heater features elite PTC technology which ensures that the heater maintains the ideal temperature range without overheating.

Hannah Fuller

Should I Turn My Heater Up/Down In The Winter/Summer?

Alisa Mendoza

Usually, you don’t need to do so, as modern aquarium heaters come with built-in thermostats. They automatically regulate the temperature and prevent water overheating. For example, if you set your heater at 80 degrees, it will sustain this temperature. The only exception is cable heaters, which are always on. They need to be regulated manually.

Julio Robertson

ISTA In-Line Aquarium Heater – Available in 150, 300, and 500 watt versions, the ISTA In-Line heater is the best choice for large aquariums. Like most in-line heaters, this unit does a great job at keeping temperatures stable while lending to a clutter-free look. The ISTA In-Line also has an auto-shutoff feature in case water runs dry in the line, eliminating any risk of fire hazard.

Pamela Mendoza

Do all fish need aquarium heaters?

Alisa Mendoza

Normally, coldwater fish, such as goldfish, can do without an aquarium heater, as they feel pretty comfortable at room temperature. As for tropical fish, they won’t survive in cool water, so, a tank heater is a must-have accessory for them.

Caleb Perez

Even though in the cold weather, you don’t face the lack of dissolved oxygen in the water like in hot temperature, low temperature is not comfortable for your fish or plants. They endure great stress during such low temperature and death can be seen. So it’s best to do something to cope up with that kind of climate. One of the easiest and most effective methods of warming up the water tank is to heat the water using an aquarium heater directly.

David Frazier

Do I Need Two or Three Heaters for mineAquarium?

Alisa Mendoza

It primarily depends on the aquarium size. For a large tank, two separate heaters can do a better job, Three heaters are sometimes used in very large aquariums.

Alicia Johnson

Orlushy Submersible Aquarium Heater. If you are looking for a submersible aquarium heater that can heat up a space of up to 80 gallons, the Orlushy Submersible Aquarium Heater might be perfect for you. This heater is made from a shatter-resistant quartz glass, which helps to keep your fish and other aquarium dwellers safe. Additionally, it can be used in both freshwater and saltwater aquariums.It’s important that your chosen aquarium heater keeps the water at a consistent temperature, and the Orlushy heater does this very well.

Guy Clark

Aqueon Pro Submersible 100W Heater is solid through and through, with durable, non-corrosive construction and even heat distribution. The heater has a range of 68 to 88° F and is accurate within one degree of the desired temperature. It’s fully submersible, can be used in fresh or saltwater tanks and automatically shuts off and resets in the event of overheating. Customers say that the Aqueon Pro Submersible 100W Heater is visually unobtrusive and reliably keeps a level temperature in their tank. With its combination of a simple setup and quality construction.

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