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Heating Your Aquarium

Well it only feels like yesterday when I wrote our article regarding ‘how to cool your aquarium,’ back in May, so I can’t quite believe the summer is over already. And what a summer it was this year in the UK - I think the weekend I put the article together was the only sunny one that we had (so apologies if I jinxed it!)!

Well this weekend the clocks went back and there’s a noticeable chill in the air. With this in mind, I thought it would be a good idea to write an article that outlines how to heat your aquarium.

The article will outline how to choose an appropriate heater, explaining how to work out the appropriate size and detailing the different types of aquarium heaters that are available, with their pro’s and con’s. I’ll also include a few tips from personal experience.

There are number of different styles of aquarium heater available:

  • Traditional Heaters
  • Digital Electronic Heaters
  • Titanium Heaters
  • External Heaters
  • Heating Cables

Traditional Heaters:

By Traditional Heaters, I’m referring to the type of aquarium heater that has been used by aquarists for the last 30 or so years. They are in effect glass tubes which include a heating element wrapped around a ceramic core. A thermostat is built into the heater which allows you to adjust the temperature setting of the heater, using the same principle of a bimetallic strip to control the switching on and off of the heater.

Traditional heaters, such as the Elite or the Juwel Aquarium heater range have been used for many years and offer the most basic form of aquarium heating. They need to be fully submerged in the aquarium with the ability for water movement to pass over the heater. If using this form of heater, ideally place it next to your filter which will help to distribute the water around both the heater and the aquarium.

A benefit of the traditional heater is that they are readily available and relatively economically priced. Unfortunately they do however have drawbacks. Their glass construction makes them fragile, so be careful when placing the heater near to rocks which may fall against the heater, or in an aquarium which has boisterous fish that may smash the heater. Heater guards are available for added protection if required.

Traditional heaters are also quite cumbersome and are likely to be visible within the aquarium. Whilst there is definitely a temptation to hide the heater, make sure that your can still see the light which informs you that the heater is operational, and ensure that you position the heater so that it can be removed/replaced with ease – if it does fail, you don’t want to have to strip down your entire aquarium to swap it!

Digital Electronic Heaters:

Digital Electronic Heaters have been available on the market for the last 4 or 5 years. Rather than using a bimetallic strip to control the heating element, a microprocessor is used. This provides significantly improved levels of control and accuracy when compared to a traditional heater. Additionally they often include a temperature gauge or lighting which indicates if the aquarium water is cooler, at the required temperature or warmer than the temperature specified.

Again these heaters should be placed near to a circulation pump to ensure the heat generated is distributed throughout the aquarium.

The main drawback with Digital Electronic Heaters, such as the TMC V2Therm or the Fluval E electronic heater ranges is price - they’re a little more expensive than traditional heaters. Personally however I’d suggest that the extra level of control and reliability of this type of heater makes it worth the extra investment (well in my opinion anyway!).

Titanium Heaters:

We stock Aqua Medic Titanium heaters which offer even greater control than Digital Electronic Heaters and can form part of a complete temperature control solution for your aquarium. Aqua Medic Titanium heaters need to be used in conjunction with an Aqua Medic Temperature Controller which can also be used to control your aquarium (Aqua Medic Titan) Chiller. Using one controller to manage both your heater and your chiller will ensure that these two pieces of equipment do not compete against one another, increasing your electricity bills.

Titanium Heaters are suitable for both freshwater and marine aquariums, and are referred to by the manufacturer as unbreakable (I’m assuming in normal aquatic applications that is!).

The main drawback of this approach to heating your aquarium is cost, however if you require a complete temperature control system for your aquarium, the Aqua Medic Titanium heater range when used in conjunction with a Titan chiller is the perfect combination.

External Heaters:

With all of the heaters mentioned so far there is one very distinct drawback, the heater has to be placed in the aquarium and lets face it, they’re not particularly nice to look at! Hydor have addressed this with the creation of the ETH external heater which can be used if you have an external canister filter.

The ETH has two hose connections (it is available in a number of sizes to suit the hoses of your external filter). It is placed inline so that clean water that has passed through your filter is pumped through the ETH on its way back to the aquarium.

There are two main benefits from this type of ‘inline’ heater. Firstly the heater is kept outside the aquarium, minimising the amount of equipment to hide in the aquarium and making it easy to access, clean, replace etc. Secondly, the water that is being heated will be circulated by the filter’s pump, helping to create an even temperature throughout the aquarium.

One drawback is you need to keep water in them! This isn’t usually a problem with an external filter, as the taps used generally keep water in the hoses whether the filter is switch on or off. It’s quite surprising how many people manage to let the Hydor ETH run dry (and left switched on), which will result in a hole the diameter of a pencil being burnt through the casing, causing a flood when you switch the pump back on! – So always leave water in your ETH, or switch it off at the mains when undertaking maintenance.

Heating Cables:

Heating Cables are designed for use in planted aquarium and should not be relied upon as a sole supply of temperature for your aquarium. They’re placed in the substrate of your aquarium, ideally when you’re first setting the fish tank up. Once the tank is filled and planted, the heating cable creates very gentle convection currents in the substrate, bringing nourishment to the roots of the plants.

What size heater do I require?

Assuming your fish tank is located in a house which is centrally heated, we recommend that you provide 1w of power for every litre of aquarium water.

If for example you have a 100litre aquarium, use a total of 100w power, if you have a 300litre aquarium, use 300w power.

You’ll notice in the examples above that I’ve not stated to use a 100w or a 300w heater, instead I’ve suggested use 100w or 300w of power.

This brings us on to the next topic:

Are two heaters better than one?

In my opinion, yes.

In this world unfortunately nothing lasts forever, including aquarium heaters. When a heater breaks it will inevitably take one of two approaches, either staying stuck in an ‘on’ position where it will not switch off and will increase the tank’s temperature; or it will not switch on at all which will cause the temperature to drop.

Obviously both of these scenarios will have dire consequences associated, however this risk can be minimised by the use of two heaters.

Take for example the first scenario, the heater has failed and is now stuck in an ‘on’ position. Say for example our fishtank is 300lts and we have two 150w heaters in it. If one of the heaters becomes stuck in an on position, it will take a longer time for the aquarium water to heat to dangerous levels with a 150w heater, compared to if we were running a 300w heater. This will hopefully provide sufficient time for you to spot that there is an issue and that the aquarium’s temperature is increasing.

If we take the second scenario with the heater failing to work completely, if you have a second heater within the aquarium the loss in temperature will be more gradual, again providing you with time to spot and rectify the problem.

As such we’d highly recommend using two heaters rather than one. Please note however that when doing this, you need to use two SMALLER sized heaters, so the sum of the wattage matches the volume of your aquarium in litres.

What if I oversize/undersize my heater?

If you use a heater that is too small for your aquarium you’ll have difficulty firstly obtaining the correct temperature for your livestock. Secondly you’ll find that you have large temperature fluctuations. For example your central heating may help to boost the aquarium’s temperature in conjunction with the heater. However when the central heating switches off, the temperature may fall and your heater may not have sufficient power to raise the temperature independently.

So should you use a larger heater just in case?

Absolutely not I’m afraid! As noted above, use 1w of power for every litre of water. If using two heaters in a 300litre aquarium, use two 150w heaters.

If you use a heater (or heaters) which have more wattage than litres, you’ll run into problems with your fishtank over heating. The reason for this is that when the heater switches on, energy is converted into heat which will be absorbed by the ceramic core of the heater together with the glass shroud. When the heater switches off due to it’s thermostat reaching the required temperature, it will take time for the latent heat absorbed to dissipate into the water. The result is that despite the heater being switched off, it will continue to raise the water temperature for some time whilst the heater itself cools down by transferring this energy into the aquarium. If for example you have a 300w heater in 100litres of water, you’ll soon notice that the aquarium water will be too warm for your livestock in no time at all.


I hope you’ve found this article useful, having run through the different approaches that are available to heat your aquarium, noting the benefits and drawbacks where appropriate. I’ve also included guidance with regards to the size of heater to use, outlined why in my opinion it is better to have two heaters rather than one; and I’ve noted the problem associated with using either an under or over powered aquarium heater.

If you have any questions or comments regarding this article then please feel free to contact us – we’d like to hear your thoughts.