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Living organisms can exist in a certain, characteristic for them temperature range. Outside this range, the normal course of life processes of organisms is impossible.
To keep fish and plants in the aquarium it is also important to maintain a certain temperature at which they live and multiply well. It is known that a change in the temperature of the medium affects the body temperature of fish and plants and leads to a change in the rate of metabolic biochemical processes in organisms, which largely affects their health and state. The rate of chemical reactions with increasing temperature by 10 degrees C increases by 2-3 times, and the acceleration of biochemical processes with heating is not unlimited. At a certain temperature, the body's activity subsides, and it is replaced by a phase of depression.
Fish that live in temperate zones of the globe are eurythermal organisms that can exist in a wide temperature range. In some cases, they plunge into winter or summer hibernation, stop eating, become immobile, burrow into the mud. In other cases, they try to escape from unfavorable conditions for them and make seasonal migrations. A sharp and violent temperature change causes a shock state, in which fish lose their ability to leave the danger zone independently, and, if conditions do not change, perish.
The amplitude of seasonal and diurnal temperature fluctuations in the reservoirs of the tropical belt is low. In this regard, the fish living in them are for the most part stenothermic species that require the maintenance of a constant, relatively high temperature in the aquarium. The usual air temperature for residential buildings is about 18-22 degrees C for many exotic species of fish is clearly inadequate. Many amateurs keep aquarium fish at room temperature, forgetting that if the room on the thermometer is 22 degrees C, then in the aquarium water is always 2 degrees C below.
The temperature difference can adversely affect the condition of heat-loving fish, most of which do not withstand temperatures below 18-20 degrees C, and fry. In this regard, when the temperature fluctuates, artificial heating of water is necessary. Electric heaters are used for this purpose.
To bring cold tap water to a tropical temperature of 24-30 degrees C, you need to warm it up and maintain it at the reached level. It should be borne in mind that the heated water without additional efforts is again cooled to ambient temperature (room temperature). Most aquariums do not have thermal insulation, and glass is not a real insulator (even organic glass is more efficient in this sense). So, to warm up the water you need two things: a heater and a regulator. Some devices combine these functions, and then they are called thermoregulators. It is equally important that the heat given out by the heater is evenly distributed across the reservoir, as the formation of different thermal zones will impede the free movement of many fish species in the aquarium. In earlier times porous stones were used for most aquariums. They contributed to the vortex movement of water, and due to this heat was distributed along the aquarium. Nowadays, this is due to the uniform action of the pump filters that drive the water.

Spiral Heaters

A traditional heater-glass tube, Within which Heating and regulating elements (In the figure, Jager devices).	Spiral heater In a metal case (titanium) Firm Schego.

The most perfect and most effective for today, among the heating devices are spiral heaters, which consist of a glass tube, a heating element (spiral) and a filler (quartz sand). But such glass tubes often break, especially if the consumer does not pay attention to the instructions for their use and when changing water does not disconnect the electric wire from the network, that is, does not remove the plug from the outlet. In such cases, the glass envelope of the device almost always overheats and bursts, even if the glass is considered "unbreakable". The question arises involuntarily: for how long will OTK permit the release of such non-heaters? In the industry, therefore, other heating devices have already been developed and manufactured, where the tube is made of a thin metal. The devices described in the next paragraph are not.
Equally convenient, but simpler in terms of manufacturing than spiral ones, are heaters in which vitrified resistance is used as a heating element.
In certain cases, especially when the temperature of fish sensitive to small fluctuations is kept, when many fish species are diluted, with significant temperature fluctuations in the room, with the difference between day and night temperatures of more than 4-5 degrees C, it is justified to use a variety of automatic systems with factory- Automatically maintaining the water temperature in the aquarium strictly at a given level. The basis of such systems is a contact thermometer, the mercury column of which reaches a certain temperature when reaching a certain temperature and closes the electric circuit of the relay. The relay, in turn, is triggered, breaks the circuit of the heating device, turning it off. However, it should be borne in mind that for most fish species it is useful to gradually reduce the temperature at night by 2-3 degrees C.
Another way is to warm up the water in the aquarium (or outside it) from below, under the aquarium bottom or under (glass) bottom. This can be done with the help of a heating cable or a board, which is placed under the aquarium (hence, its size must match the size of the bottom).

Which of the two methods turns out to be the best in practice, everyone must decide for himself. The problem is that when heating the bottom, the same question always arises: what if the heater fails? Draining the water from the aquarium so that you can pull the board out from under it and put another (if at all possible)? Dig out the heating cable from the ground at the bottom and replace it with another one? Heated bottom has advantages for plants, because The heat released by the device spreads over the soil and thereby heats the root system of plants. Following the law of physics, warm water tends to rise, and cold water descends to the bottom to warm up again, i.e. Circulation of water occurs without additional assistance. Cable heaters, which are placed inside the aquarium itself, must be of very high quality. Their shell can not be either old or fragile. In addition, you can not allow the fish to bite it. Modern cable heaters are produced in a very elastic and dense shell of silicone rubber - a material that can handle even sharp changes in temperature. And even the most inveterate kusaka will not be able to damage it.
There are heaters that are located right inside the filter. Yes, and it's possible. Different manufacturers offer their appliances for heating water in this way. For example, not unknown, and we will not hide our beloved, Eheim offers for aquariums a capacity of up to 80 liters of minifilter, which is equipped with an electric pump and suspended on the side wall: in a separate chamber there is an electronic filter for 65 watts. The filter is equipped with a built-in electronic regulator, which maintains the required water temperature. Some large (for reservoirs 450-600 liters) suction filters of the same company are also equipped with built-in regulators-heaters. For switching and adjustment on the case of the device there is a rotating handle.
As already mentioned, the water outside the tank can be heated primarily by means of a filter. But there is another method, i.e. The heating elements are placed in a separate box through which water flows. These devices have an electronic regulator and function together with a suction filter.

The data given in the table is based on calculations for aquariums of standard sizes. Since the power of heaters in this case can not be calculated with all accuracy, they need to be selected at their own discretion. Before changing water, the heater should be switched off.