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Tropical fish require a normal maintenance temperature between approx. 22-29oC (72-84oF), with many species being kept at a 'middle value' of 24-25oC (75-77oF). Maintaining a stable temperature (and more importantly avoiding rapid changes) is vital to avoid stressing fish. The temperature of a tropical aquarium can be maintained in a number of ways.

Most commonly used nowadays are rod-shaped combined heater-stats, placed inside the tank. These are available in a number of standard wattages between 25W and 300W. The table below gives examples of recommended heater wattages for various tank sizes. The modern combined heater-stats use very reliable thermocouples to maintain a stable temperature.

Heater-stat rods

External thermostats can also be used to control heating elements placed in the tank, and have the advantage of a less bulky element inside the tank.

Picture of a thermofilter

Thermofilters are external canister filters which have a heating element built into them. Many are fitted with a precice temperature controller, which may include a digital readout. Using a thermofilter avoids having an unsightly heater unit inside the tank.

Heating pads, placed beneath the aquarium, can be used to heat the base of the aquarium. It is often suggested that heating the substrate is beneficial in planted tanks as it may promote warm convection currents through the substrate to carry nutrients to plant roots. Heater cables are often employed for this purpose. These are laid on the base of the tank and substrate material (often a nutrient-containing type) placed above. Both of these substrate heating devices are normally used in conjunction with a standard heater.

Recommended Heater Sizes
Tank sizeTank volume
(Imp gal)
Tank volume
(US gal)
Recommended heater(s)
36x12x15"-48x12x15"21-2825-34200W (2x100W)
48x12x18"-48x15x18"32-4138-49300W (2x150W)
60x15x18"-60x18x18"51-6061-72400W (2x200W)
72x18x18"-72x24x24"72-12886-154600W (3x200W)

On larger tanks, it is advisable to use two or more heaters to make up the required wattage - this not only gives a more even heat distribution, but gives an extra safety margin. If one heater fails, the other heater will provide some heat and the malfunction should be noticed before the temperature drops significantly. Also, if one of the smaller heaters should stick in the 'on' position, it will not raise the tank temperature as rapidly as one larger heater.

The use of a separate thermometer is very important, and it should be checked daily to verify that the temperature is correct. Although modern heater-stats are very reliable and can be set to a specific temperature, you will need to verify initially that they are actually maintaining the correct temperature in the tank, and adjust as necessary. If you check the temperature every time you feed, you should notice any change in temperature caused by a failed heater before fish are affected.




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