Heating and Cooling

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Heating a tank is a fairly simple affair, just requiring a simple aquarium heater. The heater needs to be of an appropriate size to maintain the system at the correct temperature. A marine system of same size as a freshwater one will typically require a small wattage due to the other high powered pieces of equipment e.g. pumps and lighting. A good practice is to have multiple heaters to reach the total wattage required e.g. if tank requires a 300W heater then use 2 150W heaters. The reason for this is if one fails on or off, it is unlikely to cause any major problems before it is detected.


There are a few ways you can cool a tank:


The simplest & most expensive is to buy a chiller & leave on a tank at the set temp.

Air Conditioning

The tank can be cooled by ensuring that the room temperature is lower than the temperature at which the aquarium is to be kept. Use of air conditioners can be just as energy efficient as using chillers, and has the added advantage of this is it keeps you cool as well.

Extraction Fans

Adding Fans to hoods will also go a long way to keeping tanks cool, by extracting hot air from light fittings before it can influence tank temperature. These are also linked to the next method, evaporative cooling, by assisting in the removal of humid air, therefore increasing the capacity for more evaporation to take place.

Evaporative Cooling

This bit is really worth reading!

The effectiveness of fans for evaporative cooling should not be underestimated in its usefullness. A fan blowing onto the water surface either in the main display tank or on the sump will promote evaporation, which is exremely effective in providing a cooling effect and can pull the temperature down by 3-5ºC. Options include:

  • desk or pedalstool fans
  • bathroom exhaust fans are a very cheap option for a quiet fan that moves a large amount of air.
  • AC or DC powered computer fans - cheap, small and flexible. Attached to a variable voltage DC adapter, a DC fan can be speed adjustable and very quiet.

Be aware however increasing evaporation will increase the required replacement rate for freshwater top off and that under humid environmental conditions (eg Tropical Northern Australia) evaporative cooling is considerably less effective and other methods of cooling will need to be employed.


If the tank is small, large blocks of frozen RO water, or frozen plastic bottles filled with water placed in the tank can help in small amounts to cool the tank down. Given that this relies on human intervention, this is to be considered as an emergency measure only. It is ineffective for larger tanks due to the large amounts of ice required. As a rule of thumb 1kg (L) of ice can provide a cooling effect of around 1ºC in 100 litres of aquarium water. Consequently reducing a 400L tank by 4ºC (say from 32ºC to 28ºC) would require around 16 kg of ice. Cooling a 60L tank by the same amount would require a much more practical 2.4 kg of ice.

Prevention is Better than Cure

Even better than implementing cooling measures is to prevent the tank from heating up to start with. Most organisms will be fine for a few days without the lights turned on. This will have a huge difference if you have large lights. Locating the tank on an internal wall, away from a West-facing walls or windows, and good room ventilation will all assist in keeping the tank cool.


What is also important is reducing the amount of heat getting into the tank from its surrounding environment. This is particularly important for systems that are situation in buildings that have poor insulation, such as tin sheds, or the sump is located externally.


Traditional aquarium heaters have a thermostat integrated in the construction of the heater unit. The thermostat detects the water temperature and activates the heating element when the temperature drops below the set point.

By having the thermostat in such close proximity to the heating element, the measured water temperature can be influenced by whether or not the heating element is running at any particular time, with the result that traditional aquarium heaters have a limited ability to control temperature closer than a range of +/- 2ºC. In many situations, this is more than adequate.

A solution that enables temperature to controlled much more tightly is to use a separate thermostat with a temperature sensor located remotely from the heating element from the heater. Using such a sytem, aquarium temperature can be controlled much more tightly (say +/- 0.5ºC), leading to superior stability.

Temperature Controllers

For details, see Temperature Controller.


How many watts should my heater be?

As a very rough rule of thumb, a ratio of 1 Watt per Litre is helpful in choosing the size of heater. eg For a 300L tank, use a 300W heater (or 2 x 150W heaters). Of course this is only a starting point. In warmer climates, or during summer, you may not need a heater at all. In colder climates, or in winter in a poorly insulated room, this estimate may prove to be insufficient.

Also consider that lighting and water pumps can also contribute significant amounts of heat to the aquarium, and may reduce the amount of suplementary heating required.