How to choose an aquarium heater

by Owen James

There are places in the world where they keep tropical fish without worrying about heating the water. Such places are called ‘the tropics’.

The rest of us need to choose an aquarium heater that will keep tank water in the range preferred by our fish: usually between 20-30 ºC, with 24ºC being suitable for most common tropical fish.

Note that even if you heat your house, an aquarium heater is usually required. That’s because the aquarium water doesn’t just need to be warm, the temperature should also be as stable as possible.

Fish, particularly more delicate ones like marines and discus, are as stressed by fluctuating temperatures as they are by water that’s too warm or cold. You don’t want the temperature to dip at night and then rise by day, for instance.

Aquarium heaters must therefore use thermostats to keep the temperature constant. The thermostat may be a separate device that you connect to the heater or, far more commonly these days, it can be built into a single device with the heater.

What size aquarium heater?

Aquarium heaters are rated in watts. Manufacturers also state on the packaging what size tank a particular heater is designed for.

The rule of thumb is that you require five watts per gallon to heat one gallon of water. So a 20-gallon tank would require a 100-watt heater.

The five watts per gallon rule is pretty conservative, especially with large tanks in centrally-heated homes. (Larger tanks lose heat more slowly than smaller tanks). If you have a 30 or even a 40-gallon tank, unless your house gets very cold at night you’ll find a 100-watt heater will suffice.

For larger aquariums it’s better to have two heaters, and to split the wattage. So for a 50-gallon tank, you might have two 100-watt heaters, say. The reason is that if the thermostat fails and the heater sticks ‘on’, you hopefully won’t boil all your fish before you notice a heater is broken. (I’ve over-heated a small tank of fish, and it is a dispiriting experience indeed).

Some new heaters include an alarm to warn you when they stick ‘on’, or you might notice from strange fish activity such as gasping at the surface, or from the thermometer of course. If your heater does fail, turn it off but don’t remove it right away as the glass can get very hot and may crack over your fish tank.

Types of aquarium heater

There are several types of aquarium heater, although nearly everyone now uses the standard combined heater/thermostat units, which are cheap, fairly accurate and very convenient. I’ll consider some more specialized heaters below, too.

Combined heater/thermostat

These are long thin glass tubes, containing the heater at the bottom and the adjustable thermostat at the top. The whole unit is designed to be submersed in your aquarium. The unit can crack if it’s switched on without water, and it can also get too hot to touch, so take care when cleaning your tank – even though you’ve switched the power off first.

The heater is always a coiled element within the tube, though some modern units make the tube out of toughened plastic rather than glass. Tough plastic is less prone to cracking or burning aquarium inhabitants.

The thermostat may be an old-fashioned metallic strip element, or a more modern microchip-based unit. The former have stood the test of time, though the latter may be more accurate – at least until the chip fails! If a microchip thermostat should fail, the chip is meant to fail in the ‘off’ position, giving you plenty of time to install a spare heater when you notice the tank cooling. Warming water quickly is a much more dangerous situation.

Combined units invariably have a light at the top to tell you when the heater is working. Position the unit so you can see this light when the heater is on, so you can check things are working as they should. (e.g. If the tank is too cool, the light should be on to show the heater is working).

Combined heater/thermostats are the best choice for 95% of aquariums.

Separate thermostat

Sometimes you may want to heat your tank the old-fashioned way, using a separate heater and a thermostat. For instance, you may be using an undertank heater or you might want to use a smaller heater in your aquarium, keeping the bulky thermostat outside of the tank.

External thermostats are easier to adjust, and your hands can stay dry. The combined cost will be more expensive though.

Undertank heater mat

These became quite popular in the 1990s, but I don’t recommend them. The theory is great: you place your tank on a heater mat, which warms the tank through the glass. The temperature is controlled via a separate thermostat. An ugly thing is removed from your fish tank.

Unfortunately, glass is a bad conductor of heat, and gravel on the bottom of the tank doesn’t help with  circulation at all. This can make using these units rather erratic. Worse, if the heater fails you’ll need to drain and move your tank to replace the heater.

Best left for keepers of tropical reptiles and amphibians, in my view.

Heating cable

These don’t replace normal heaters, but rather are an addition to help grow plants. They are thin cables that you snake through the aquarium gravel. They are usually fixed in place with suckers before the substrate is put on top.

The idea is that the gentle local heating promotes water movement through the aquarium gravel, stimulating the roots of plants just like springs in nature, and helping with fertilization.

Heating cables were very popular a decade or two ago with planted systems from the likes of Dernelle and Dupla. However many modern aquascapers (including the Japanese planted tank master Amano) don’t use them.

Heater cables may be worth experimenting with if you’ve got several tanks, but expensive and certainly not essential for plant growth.

In-line heater

If you use an external filter, you can buy heaters from manufacturers like Hydor that you plumb into the tubing between your external filter and your tank. As water runs through the heater it’s warmed up, which provided you use a sufficiently powerful filter pump can be an efficient way of heating the tank.

The external casing of an in-line heater is usually made of heatproof plastic, so it’s safe to hold, and won’t start fires. But the bigger advantage is that you’ve got the heater out of the aquarium, which could be good if you’re aquascaping or keeping fish that could burn themselves (some big catfish or seahorses, for example) or that will even fight with equipment (certain cichlids in freshwater tanks, and triggers in marine tanks).

In-line heaters are quite new, however, and more expensive than normal units. There’s also a danger with anything plumped in-line that it can come unplumbed. Every extra cut in the tubing is another opportunity for leaks.

External heater/filter (thermofilter)

Some external filters combined filtration with heating in the same bit of kit. Water passing through the filter is also heated, with the temperature controlled by settings built on top of the filter canister.

The advantage with these units is no extra plumbing, and no danger of the leaks we just mentioned. Disadvantages include the cost – more than the equivalent filter plus even an in-line heater, let alone a conventional heater/thermostat – plus the fact that if the heater fails you’ll have to discard the whole device.

The most popular make, the Ehiem Thermofilters, have a good reputation for reliability, but everything man-made will fail someday. Also note that if you keep marines, make extra sure the heater/filter is suitable for saltwater use. Some aren’t.

Other things you’ll need when heating an aquarium

A thermometer: This is essential, to enable you to monitor the tank temperature and to look out for heater failures.

Spare heater: Always have at least one spare heater in the house, in case yours fails. Even if you live minutes from the local fish store, you can bet your heater will fail on a Sunday, or at midnight when it’s closed.

A heater guard: These are cheap plastic guards that fit over the heating element of a conventional heater. They are said not to affect its performance, and they will stop some fish burning themselves on a very hot unit. I’ve used them in seahorse tanks, since seahorses will wrap their tails around anything they can. May be worth the small amount of money they cost.

Check out Katy's Tropical Fish Guide for more aquarium info.

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{ 2 comments… read them below or add one }

Rob 07.21.11 at 11:25 pm

Can i use a heater that is meant for a 2 to 5 gallon tank, on a 15 to 20 gallon tank?

Owen James 12.20.11 at 1:52 am

@Rob — Yes. If you’ve got a centrally heated house you might be okay, but if your house gets cool get a properly sized heater.

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