Fish are cold-blooded, or to use the correct term poikilothermic. That is, they take their body temperature from the surrounding water. As all the fish featured on this site are from tropical regions, we must try to mimic the temperatures associated with their natural environment in order for them to remain healthy.

Most tropical aquarium fish are content with a temperature range of 70°F to 82°F and 76°F is usually a happy medium, but always research the requirements of the species you intend to keep as some species like temperatures far outside these ranges.

In the wild, the temperature of a body of water will vary with the seasons and to a lesser degree (no pun intended) with the time of day. The important point to remember is that sudden changes in temperature do not occur in nature and the same consistency to temperature control should be applied in the aquarium. Nature's exception is unseasonably cold weather when occasionally millions of fish die. Perhaps indicative of the effect of rain, changes of ± 4°F in an hour can be tolerated by most fish but constantly fluctuating or rapid changes cause stress, induce disease, and at worst death. Prolonged exposure to low temperatures can make fish lethargic and refuse to feed. Conversely, high temperatures increase their metabolism and can adversely effect their colouring, fertility, and life span. Short term exposure to warmer temperatures (imitating a seasonal change) is however sometimes applied as a method of inducing spawning and used as a means of disease control - parasites are often intolerant of higher temperatures.

The need to maintain a constant temperature in the aquarium therefore necessitates artificially controlled heating.

There is a range of heating devices available to the aquarist and all consist of three basic components:

• A heating element (heater), to physically warm the water.
• A sensor, to monitor the water temperature.
• A thermostat, to switch the heating element off or on when the sensor detects a maximum or minimum temperature.

Glass immersion heaters are one of the most common types of heating element. They consist of ceramic or glass rods wound with a heating coil and are encased in a glass tube. Electrical resistance warms the coils that in turn heat the water. Control of the heater is provided either by a combined thermostat or a separate, external thermostat unit.

Immersion heaters are designed to be mounted inside the aquarium. They should be positioned so that there is a constant flow of water around them to distribute heated water away and draw cooler water across the element. Although inexpensive, glass heaters can be damaged easily. Care should be taken not to knock them and always switch them off at least five minutes before removing them from the tank or performing a water change as they can crack if suddenly exposed to a cooler environment such as air or cold water. Similarly, do not switch these heaters on when they are outside the tank as they will overheat. One other potential cause of damage to these heaters can be the fish themselves. Boisterous fish have been known to smash the casings of glass heaters. On the other hand, as these heaters produce a high heat source over a concentrated area, they can reap revenge by scalding fish that rest on them. Catfish are particularly prone to this (remember fish are poikilothermic and cannot sense they are being burnt). Plastic guards are available to protect against this they are however unsightly.

These problems can be resolved by using a heating cable buried in the substrate or an external heat mat similar to those used in terrariums. Installing these heating devices is only practical when setting up a tank and not worth the effort in an established aquarium. The main advantages of heat mats and cables are they provide good heat distribution since the heating element is spread across the entire base of the tank. They are also beneficial to encouraging plant growth. Convection currents, created by warming the substrate, slowly circulate nutrients from within the substrate and the aquarium water to the plant's roots and carry away exhausted or waste compounds. Additionally, as water does not normally flow particularly well through the substrate, it is usually a degree or two cooler than the main body of the aquarium and this can inhibit plant growth. Heating the substrate provides plant roots with "warm feet" encouraging growth.

It is important to choose carefully the heat mat or cable you intend to install. These heaters are often, but not exclusively, designed only to warm the substrate to promote plant growth and therefore are not of sufficient power to heat the aquarium water as well. Usually they are used in-conjunction with an additional source to heat the aquarium water.

Attempting to heat the aquarium water an over powered cable or heat mat can have two drawbacks. Firstly, hot spots on the base of the aquarium can be created, particularly when compact materials such as sand or large stones are used as part of the substrate. These items heat up, do not dissipate heat well, and can cause the base of the aquarium to crack. A heat mat or cable that becomes too warm may also draw water too quickly past the plants roots effectively stripping away the very nutrients that the convection currents they create are intended to carry.

I like the fact that these heaters are hidden and do not influence the fish's habitat. They do have one major disadvantage, once installed they cannot be removed without major upheaval, and should they fail the simplest solution is to leave them in place and install a glass immersion heater instead.

Thermofilters are a relatively new innovation and take the heater out of the aquarium altogether. As water is passed through the filter, it is heated to temperature before being returned to the tank. To emulate this principle, cheaper glass immersion heaters could be placed in a sump filter rather than in the tank. A benefit of this is any adjustments to the heater can be made without disturbing the aquarium and again even heat distribution is assured.

A temperature sensor is required to detect when the water temperature reaches a maximum or minimum level and initiate heat control via the thermostat. The high tech approach is to use an electronic sensor housed within a probe. Some probes can be inserted discretely into the tank and can accurately monitor the water temperature to within a fraction of a degree. Others sense the temperature externally through the aquarium walls but are sometimes adversely affected by the room temperature and children! Typically, the sensor will be combined within the thermostat and this will be in the form of a microchip or more commonly, a bimetallic strip.

Bimetallic sensors are composed of two metal strips attached back to back that expand and contract at different rates when heated or cooled. When heat is applied to the strip the different expansion rates of the metals cause it to bend outwards and break the heating circuit, as the strip cools the strip bends inwards and the circuit is completed allowing heating to continue. Bimetallic strips are fairly accurate usually maintaining temperature within ± 2°F.

Like sensors, thermostats can be internal or external from the tank. Sometimes one external thermostat can be used to control a number of heating elements and this is particularly useful (and cost effective) when heating 2 or more tanks. One drawback with this system is if the thermostat fails in any way all tanks will be affected. Another factor worth considering here is that different tanks will heat and cool at different rates. Aquariums containing many rocks for example will take longer to heat and to cool and thus some tanks in the system may not be maintained at the desired temperature.

Combined heaterstats (pictured top) encompass a thermostat, sensor, and glass immersion heater in one unit. Some heaterstats are pre-set but most have an adjuster with a scale to allow the user to fine-tune the required temperature. Usually these units are designed to be completely submerged in the aquarium but some should be positioned so that the top of the heater is above water level. Always refer to the manufacture's instructions for installation advice.

Heaters are available in a range of wattage's, the higher the wattage the greater the heat it will output. To determine the heater wattage required for your tank we suggest using approximately 1 Watt per litre of water (approximately 5 Watts per Imperial /US gallon) and round up to the nearest heater size. Use the Tank Volume, Weight and Heater Wattage Calculator to determine the heating requirements of your tank. The amount of heating may vary depending on the ambient room temeprature. If you live in a particularly cold climate, it maybe worth investing in a more powerful heater. Conversly, in warmer regions, the ambient room temperature may be sufficient year round to warrent the purchase of a low Wattage back up heater.

In the UK, heaters are sold according to their output in Watts and standard heater wattages are 50W, 100W, 150W, 200W, 300W, and 600W. By example, a 30 gallon tank would require 120 Watts and therefore you should use the next highest standard heater size, which would be 150 Watts. The heater wattage makes little difference to its price therefore; it is always better to slightly overstate the wattage required avoiding chilling the tank in cold weather. High wattage heaters warm the water quicker for the same cost and because they do not have to switch on and off as frequently as smaller heaters or work as hard to heat the water, they are less prone to failure. In larger tanks, the required wattage should be spread between 2 or more heaters. This will give improved heat distribution throughout the tank and in case of the thermostat sticking in the "on" position, there is less likelihood of the tank overheating. Similarly, if one heater fails a second will still provide some heat as a back up.

Aquarists with multiple tanks often keep them in a specially designed room called a "fish house" and here the most economical method of heating is to heat the room rather than installing a separate heater / thermostat each individual tank. Since hot air rises, fish with different temperature requirements are usually kept at different levels within the room. This poses a small problem for specialist breeders who may like to keep species with the same temperature requirements. The solution is to use fans to circulate warm air evenly around the room. Fish houses need to be exceptionally well insulated to minimise heating costs, but I am told a fish house measuring 12 feet by 6 feet by feet can cost as little as a few Pounds a week to heat. As with any warm room with a quantity of water in it, take an indoor swimming pool as an example, condensation can be a problem and the room must be suitably protected against dampness. 

Whichever method of heating the tank you decide to employ, you will need a thermometer to monitor the temperature. These are discussed here.