Available devices for water heating in an aquarium.
Heating equipment consists of a heater that warms the water, a thermostat that maintains a constant water temperature by turning the heater’s power supply on and off, and a thermometer.
About heater types:
1. Heaters — the simplest devices in the form of a sealed glass or metal (stainless steel) tube-bulb, inside which a heating element is mounted as a spiral of nichrome wire. Temperature control is not provided in such devices. Such simplest heaters are increasingly less used by aquarists and are becoming increasingly rare on sale.
2. Electromechanical thermostats — technical devices combining the functions of a heater and a temperature controller mounted in a single housing — a glass or, again, stainless steel tube. The temperature-regulating element in them is a bimetallic plate that closes or opens contacts when heated or cooled. Most commonly used in aquarium practice. As is known, warm layers rise upward, displacing cold ones. Therefore, it is not necessary to place such devices vertically, otherwise the thermostat will turn off prematurely. An inclined or horizontal position is best.
3. Electronic thermostats. In these, instead of a bimetallic plate with contacts, an electronic circuit is built into the heater. As a rule, they are supplied only complete with a heater immersed in the aquarium. They have a display showing the water temperature and an audible alarm for aquarium water overheating.
4. Heating cables. Designed for installation at the bottom of the aquarium within the substrate layer. Heating cables initially carried the idea of substrate heating for a beneficial effect on plant growth. They are as useful for plants as heating mats. Here, the aesthetic side of heating organization undoubtedly wins. Removing a cable is easier than a mat, but the aquarium interior is heavily disrupted in the process. Moreover, heavy decorative items, such as large rocks, can damage the heating cable.
5. Heating mats or pads. Placed under the aquarium or terrarium, heat rises upward through the bottom of the aquarium. According to some aquarists, warming the substrate has a favorable effect on plants. The main disadvantage of heating mats is that it is impossible to remove the heater without removing the aquarium itself. However, their lifespan is much longer than that of coil heaters.
6. External inline thermostats. Mounted in the water circulation line of an external filter. Recommended for use in fairly large aquariums. The product line does not include inline thermostats with a power of less than 200 W. A good, clean method of heating. These devices are quite reliable and feature an electronic thermostat.
7. External canister filters with a built-in thermostat. Thermofilters. This type of heater is structurally part of the canister filter, controlled by a built-in microcontroller, and has a digital temperature indicator. Such devices are quite reliable, convenient, and durable. Temperature maintenance accuracy is high. However, water flow issues can lead to an unacceptable drop in aquarium water temperature. In addition, depending the entire “life support” system on a single fuse and electrical appliance is undesirable.
8. Mounts and protective guards for heaters. In most cases, suction cups serve to fix heaters in the aquarium; it is better to use silicone suction cups, the lifespan of which is measured in years. The protective guard is designed to prevent damage to the glass bulb of the heater, contact of the heater housing with the aquarium material (acrylic or glass), and to protect fish from burns. The guard, equipped with suction cups, simultaneously serves as a mount.
Nowadays, the most common type is a device combining a heater and a thermostat. Such a device is immersed in the aquarium. Sometimes the heater and thermostat 
are purchased separately. In this case, replacing a failed heater will cost less. However, a heater combined with thermostats is more convenient. Heaters are produced in various capacities. It is important that their total power is sufficient to maintain the required temperature in the aquarium when the room temperature reaches its minimum value (and consequently, heat losses in the aquarium are at their maximum) — for example, on a cold winter night. It would be wise to distribute the required power across two heaters. Then if one of them fails, the aquarium will not be left without heating. However, in small aquariums, this is practically unfeasible. Two sets, each consisting of a heater and a thermostat connected in series, have an additional advantage: if one thermostat is faulty and turns on heating when it is not needed, the second thermostat will remain off, reducing the likelihood of severe water overheating.
Internal water heaters — water heating devices that do not have their own thermoregulation system. To ensure the specified water temperature in the aquarium, additional equipment — an autonomous thermostat — must be installed. Heaters — the simplest devices in the form of a sealed glass or metal (stainless steel) tube-bulb, inside which a heating element is mounted as a spiral of nichrome wire. Temperature control is not provided in such devices. Such simplest heaters are increasingly less used by aquarists and are becoming increasingly rare on sale.
Electromechanical thermostats — technical devices combining the functions of a heater and a temperature controller mounted in a single housing — a glass or stainless steel tube. There are two main types of thermostats: • Electromechanical thermostats using a bimetallic plate. The temperature-regulating element in them is a bimetallic plate that closes or opens contacts when heated or cooled. In devices combining a heater and a thermostat, a bimetallic plate is used as the actuating device in most cases. Since heat rises from bottom to top, this type of heater should not be placed vertically — otherwise the thermostat will turn off prematurely. Instead, it is better to place such devices horizontally or inclined.
There are also external electromechanical thermostats — they “sense” the temperature in the aquarium through contact with the glass. • Electronic thermostats. In most cases, they are installed outside the aquarium. They are equipped with a temperature probe that is lowered into the water. These are accurate, reliable devices that last a sufficiently long time. Aquarium thermostats most often have a limited temperature range corresponding to what is typically required for tropical fish. The operating temperature is set by rotating a dial or screw.
Sometimes the thermostat has a temperature scale (which may have varying accuracy). In most cases, thermostats, including those combined with a heater, have a light indicator that turns on when electric current flows to the heater. However, some indicators glow constantly, only showing that power is being supplied to the device.
The traditional aquarium heater is a glass tube resembling a test tube, inside of which a heating coil is wound onto a ceramic rod support. To ensure 
all parts are held tightly, the glass tube is usually filled with sand after installing and fixing the heating elements. But such glass tubes often break, especially if the user ignores the usage instructions and does not disconnect the power cord from the mains during water changes. In such cases, the glass shell of the device almost always overheats and bursts, even if the glass is considered “unbreakable”. This is precisely why industry has already developed other heating devices where the tube is made of thin metal and the heaters are enclosed in an ultra-sturdy housing. To protect aquarium inhabitants, the heater is equipped with protective grilles.
Equally convenient, but simpler in manufacturing than coil heaters, are heaters that use a glass-enclosed resistor as the heating element. The temperature can be precisely set in the range of 18–34 °C. Each click of the controller raises or lowers the temperature by +/-0.5 °C. The precision thermostat accurately maintains the temperature value, preventing sudden changes in aquarium water temperature.
Stoppers at the upper and lower ends of the scale prevent accidentally setting a temperature that is too high or too low. The special design of the device ensures its complete tightness. An outer silicone coating and inner rubber seals guarantee protection against leaks. Made of high-quality reinforced rigid polymer and aluminum, the device is practically unbreakable.
There are heaters located directly inside the filter — thermofilters. The heater warms the water as it passes through the filter. If we are also willing to incur additional expenses, then it makes sense to purchase an external filter with a built-in heater. Thus, we solve the problem of how to disguise the heater with plants and don’t have to worry about breaking it during water changes. Various manufacturers offer their own devices for heating water in this way. The filter is equipped with a built-in electronic controller that maintains the required water temperature. Some large intake filters from the same company are also equipped with built-in regulator-heaters. For switching and adjustment, there is a rotating knob on the device housing.
But there is another method, i.e., heating elements are placed in a separate box through which water passes. These devices have an electronic controller and function together with an intake filter. External inline thermostats. Mounted in the water circulation line of an external filter. Recommended for use in fairly large aquariums. The product line does not include inline thermostats with a power of less than 200 W. These devices are quite reliable and feature an electronic thermostat.
External canister filters with a built-in thermostat. This type of heater is structurally part of the canister filter, controlled by a built-in microcontroller, and has a digital temperature indicator. Temperature maintenance accuracy is high. Thermofilters are characterized by greater ease of use and a more advanced design. The heating element is located in the bottom section of the filter, so heated, filtered water flows back into the aquarium. The temperature controller is located externally in a convenient place for use.
There are heating mats that can be placed under the bottom of the aquarium. However, this type of heater is gradually being abandoned because a relatively large amount of heat goes to waste, as the heat is mostly absorbed by the stand, the bottom of the aquarium, and sometimes even the frame itself before it reaches the aquarium inhabitants. Heating cables or wire-heater elements inside the aquarium are more efficient. They are laid under or on the bottom of the aquarium during its initial setup, covered with decorative material, and secured. There are also heating systems located directly on the bottom under plastic protective grilles.
Heating cable — a flexible high-temperature heating cable made of a high electrical resistance alloy in a heat-resistant insulation sheath for heating water, air, various gas media, metal products, and other materials. To ensure the specified water temperature in the aquarium, additional equipment — an autonomous thermostat — must be installed. Heating cables and plates contain a nichrome wire inside, which acts as the heating element — similar to the coil of an electric heating pad. But unlike a water heater, the heat is not concentrated in one place, but distributed along the entire length of the cable. Such cables are laid in the substrate to heat it.
Thermostats. Some of them operate using a thermocouple — they cannot always be relied upon, but they are inexpensive. Such regulators are usually built directly into the heater tube or located in a separate tube. Electronic thermostats — instead of a bimetallic plate with contacts, an electronic circuit is built into the heater. Many variants of electronic controllers have been developed, and their cost varies so much that the device’s efficiency degree can be estimated from it. Useful regulation devices are those that not only provide the set temperature but also simultaneously display the actual temperature on a digital indicator.
Any regulator must certainly be equipped with a sensor that measures the actual temperature. And if the electronic thermostat is located externally, this device requires an additional element that must be located in the aquarium water itself. In certain cases, especially when keeping fish sensitive to small temperature fluctuations, breeding many species of fish, during significant room temperature fluctuations, or when the difference between day and night temperature exceeds 4–5 °C, the use of various factory-made automatic systems with thermostats that automatically maintain the aquarium water temperature strictly at a set level is justified. The basis of such systems is a contact thermometer, the mercury column of which, upon reaching a certain temperature, reaches a contact and closes the electrical circuit of a relay. The relay, in turn, activates and breaks the heating device circuit, turning it off.
Using a heater in the aquarium. Heater power, Thermostat.
Temperature conditions for keeping fish and plants. 
