Aquarium thermometers.
Always use a separate thermometer, even if the heater has a temperature indicator. The following types are used in aquariums: mercury, electronic, liquid crystal, and alcohol. • Mercury thermometers have very good accuracy, but breaking a thermometer inside the aquarium will lead to the death of fish and possibly require the destruction of the entire contents. • Electronic thermometers are very accurate, but unjustifiably expensive. Some of them sound an alarm when the temperature drops or rises beyond specified limits. • Liquid crystal thermometers are a strip with a color scale applied. They are glued to the outside of the glass. The temperature is shown by a more “bright” highlighting of the color square opposite the corresponding temperature indicator. Not very accurate, but very unpretentious.
• Alcohol thermometers are more accurate than liquid crystal ones, but less than mercury and electronic ones. Thermometers, especially alcohol ones, deteriorate over time and do not show the actual temperature. • Thermometers with a heat-sensitive metal coil, liquid crystal thermometers, and mercury thermometers are more durable. When buying, preference should be given to thermometers with a brightly colored column, which is better visible in water.
The thermometer is installed in the middle layers of water on the wall opposite the heater. Thermometers with weighted tips that stand vertically in the water are more convenient. Usually 
they are attached to one of the aquarium walls on brackets or rubber rings with suction cups; they tend to detach – either on their own or with the assistance of overly inquisitive fish. Unfortunately, thermometers, especially sadly alcohol ones, become inaccurate over time. In this regard, it is recommended to use mercury thermometers with a graduation from 0 to 40 degrees, but only periodically immersing them in water when measuring temperature.
In large aquariums, measuring the temperature with a floating thermometer makes no sense for the simple reason that the water near the bottom will always be colder than the water near the surface. Liquid crystals are activated at a certain temperature, or the numerical temperature or a bar sliding along the scale comes to the foreground.
Liquid crystal thermometers that are glued to the outside of the aquarium glass do not stand up to criticism at all, because they are too inaccurate in their readings. This is more of an indicator than a temperature measuring device, since it shows the temperature of the aquarium glass, rather than the bulk of the water. What can it show with a glass thickness of 10-12 mm? The thicker the glass, the greater the error.
External sticker thermometers measure the average value between the temperature inside the aquarium and the room air temperature. If the aquarium is in a heated room, this is hardly noticeable. It is undesirable to dip liquid crystal thermometers in water and glue them from place to place, as this will lead to damage.
The digital thermometer is mounted using two “Velcro strips” on any smooth surface outside the aquarium (not necessarily on the aquarium itself). The heat-sensitive sensor is fixed in the water with a rubber suction cup with a deep funnel. The nearly meter-long connecting cable will solve problems with placing the digital thermometer. Power is supplied by a single AAA battery, which ensures the device functions for at least half a year.
Controlled by a single button. Successive presses allow you to record the temperature both inside and outside the aquarium. Prolonged (over 5 seconds) holding of the button pressed turns off the thermometer (this saves battery life). Undoubted advantages of the digital thermometer include a large liquid crystal display, high measurement accuracy (up to tenths of a degree), and stability of readings over a long period of time.
There is a wide assortment of thermometers for sale. When buying a thermometer, ask the seller to place several thermometers of your choice on the counter and, after a couple of minutes, compare their readings. The spread is obvious, especially in alcohol thermometers most commonly used in aquariums (up to 1°C). The method of averaging their readings allows you to choose the most accurate thermometer.
If any of them deviates from the others by two degrees or more, it is likely lying. If the readings of all of them fluctuate wildly, it is probably a poor-quality product. The ideal option is to check the accuracy of the thermometer by comparing its readings with a laboratory or other accurate thermometer, and subsequently making appropriate corrections when taking readings from the aquarium thermometer.
The use of ozone in the aquarium. Bactericidal action of ozone.
Heating cables. Bottom heating, Application procedure. 
