The best external inline aquarium heaters

If you’re in a hurry and eager to find out the best inline aquarium heater. Then we would recommend the DaToo Aquarium External Heater in-Line Fish Tank External Heater.

When setting up your fish tank, thanks to advances in modern aquarium technology, you can now set up a realistic-looking fish ecosystem using an external filter and inline aquarium heater. Everything necessary for a balanced aquatic environment can now be completely hidden away from sight.

Fishkeeping: the early years

Often when setting up a fish tank, a big concern was trying to hide the myriad of tubes, cables, and wires coming out of the filters, heaters, and thermostats. Pretty much everything you needed to run the system was inside the tank. Except of course, for the vibrating air pump that drove the under-gravel filters. No matter how you tried to deaden the sound, you could hear it droning away day and night to keep the filter working and the fish happy. 

As tropical fish keeping became more popular, aquarium technology began to improve. Inside (tank) filters and external filters were developed, and you could throw away the infamous, noisy, vibrating pump. As inside filters had to be a specific size to purify the water properly, they were difficult to hide. So external filters, although more expensive, gained in popularity. 

Strangely, up until the last few years, little thought was given to heaters and thermostats, which still needed to be submerged in the tank. After all, they are there to heat the water; how will they do that, if they’re not in the wet stuff?

The downside of using internal heaters

Although it’s never a good idea to mix electricity and water, submersible heaters/thermostats have been reliably heating tropical fish tanks for decades. In my years of fishkeeping, I have had a few let me down, usually from the heating element or thermostat failure. I know of instances where the glass tube has been broken by heavy tank décor falling on them, over-aggressive tank cleaning, or seals failing, allowing water to enter the tube.

The majority of these failures were more annoying than anything else, but in one instance, when a friend put his hairy arm into the tank to lift the broken heater out, an electric shock was the result. Luckily, not fatal.

You can’t bury an internal heater in the gravel to hide it. It needs a good flow of water around it to stop the glass tube from getting hot and shattering. You can’t fit it upright in a corner to try and hide it either. The warmed water will rise to the thermostat, causing it to switch off. In this scenario, the tank will fail to reach the required temperature causing cold spots in the tank, and the fish will suffer. The internal heater/thermostat should always be fitted horizontally or at an angle. Usually around 45°, and on the back of the tank.

Hiding an internal heater

Every hobbyist has their idea of how they want to decorate their fish tank. If young children are involved, think colored gravel, plastic plants, mermaids, and shipwrecks. In that situation, trying to hide the heater is not the primary consideration.

My personal preference is to produce a realistic underwater world of live plants and bogwood if it’s for a community of Rasboras, Tetras and Discus. Or crushed coral, sand, and chunks of slate for communities of Malawi cichlids. For safety, I often stick the large pieces of bogwood or slate together, and to the sides of the tank, to minimize the risk of it falling and breaking the heater—an ever-present risk with Cichlids, who are notorious diggers of the substrate.

As you can imagine, with the two setups I just explained, with much of the decoration sealed together; a heater breakdown would create significant upheaval in the tank. Often, half the decoration would have to be removed to reach and replace the failed heater. With all this in mind, hallelujah, came the creation of the external inline heater.

External Inline heaters: how they work

An external inline heater should be used with your external filter. The heater is fitted inline, on the filters return flow tube to your tank. As the filtered water flows through the pipe, it’s heated to the required temperature by the sealed inline heating element, before being returned to the aquarium. The thermostat’s digital display gives you a water temperature read-out, and most importantly, can be adjusted without putting your hands inside the tank.  

needyfish’s top external inline aquarium heaters

1. Hydor ETH 200 In-Line External Aquarium Heater 

Key features

  • Suitable for marine and tropical aquariums
  • Come in 3 sizes 
  • High precision electronic temperature control
  • 5/8 inch hose

Like most inline external heaters, The Hydor ETH 200 is designed to be used with an external canister filter. It’s marketed as the first external heater to use Positive Thermal Coefficient (PTC) technology, an additional safety feature designed to shut down before overheating. People were very impressed with the ease of installation. It’s worth noting that you will need to make sure your filter has the right sized return tubing to fit the heater connections. Most importantly, when connected inline, the heater must hang vertically, to ensure water passes around all areas of the unit, so no air bubbles build up. Although the majority of users never had any issues with this heater, some people have reported early failure. I believe this to be because of the heater not being securely fixed in an upright position. Adjusting the temperature will involve trial and error, but when set it is very accurate. I would recommend double-checking with an aquarium thermometer for the first day or so.


  • Brand with a proven track record
  • Easily fitted to your external filter
  • Good temperature accuracy
  • Adequate safety features


  • More expensive than traditional heater/thermostats

2. ISTA – In-Line External Aquarium Heater 

Key features

  • Comes in 5 sizes 
  • Auto power off safety system

This external heater is a popular model with tropical fish keepers, partly due to the variety of hose connections included, its accuracy, and its built-in safety features. To set it up, you will need to fit it to your external filter’s return tube and make sure it’s fixed in the vertical position. The unit has two temperature sensors and a dual digital display. One shows the operating temperature, and the other the water temperature. If the difference between the two exceeds 5°C, the heater will shut down. If the heater tops 36°C or runs dry, it will also switch off. The digital read-out will also flash one of three codes, to warn of different operating problems. 

Some users didn’t like the fact the display is only in Centigrade, as opposed to Fahrenheit. And some had problems with the hose connections that sometimes caused leaks when connecting up. All-in-all an excellent aquarium inline heater worth the premium price.


  • Even temperature distribution throughout the tank
  • Excellent safety features
  • Flashing display provides visible warning of a fault


  • Filter reliability is a must to retain aquarium temperature

3. Datoo in-Line External Aquarium Heater

Key Features

  • Leading Temperature Control System, complete control for range 68 ℉ to 95 ℉ range
  • Safe Over-Heat Protection, two temperature sensors
  • Comes in 3 sizes (outlet and inlet heads)
  • Includes a cleaning brush
  • One Year Warranty

This is a well tested and trialled product, one obvious thing to point out before we start, is this is solely applicable for use with a canister filter. A number of key areas jump out, it is very easy to set up and if you are using this for a 100 + gallon tank, the tank is quick to warm up and the temperature control is consistent and easy to maintain throughout the tank. This consistency of temperature has been highlighted, with most users commenting that you don’t have to worry about hot spots or cold spots forming within the tank due to the quality of the heater. One user highlighted again the ease of use, giving 5 stars and commenting, “just set and forget,” as a fish owner this is what we want, ease and confidence in a product. The final area to mention is this product doesn’t leak and the two sensors mean you can be certain, you aren’t going to accidentally cook your fish, the safety of this product is highly recommended.


  • Sturdy and Durable
  • Value for money (not the most expensive product out on the market)
  • Very easy to set up and run “just set and forget”
  • Temperature is consistent and easy to maintain


  • If I was being picky (one reviewer said the packaging was slightly damaged in delivery)

Our choice

The joy of being able to start decorating a new set-up, without having to consider where and how to hide the heater, or any other necessary accessories, makes a big difference to the enjoyment of fishkeeping. 

You will never again spend hours planning how a new tank is going to look and then further hours making the idea a reality, only to strip half the decoration out a week later, to replace a faulty heater. A situation that’s occurred more often than I care to remember.

For me, external inline heaters are the way to go for any display tank. Of those I’ve tried, the DaToo has to come out on top, if only for its safety features. While the Hydor comes best of the rest. If you’re setting out on your tropical fish keeping journey, it’s worth spending that little extra, and begin planning how your fish ecosystem is going to look, without the clutter of internal heaters.

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