Thermometers that are attached to the outside of the aquarium pane are often sold as digital thermometers. These are specially coated plastic strips that change color depending on the temperature. The display accuracy is usually 1°C.
Thermometers attached to the outside of the aquarium pane measure the temperature of the pane, not the temperature of the aquarium water. Because when the temperature of the water changes, e.g. B. when changing the water, the disc changes only slowly and with a delay, these thermometers are correspondingly inaccurate. Shown is an average temperature generated over a larger area and over a longer period of time.
Proper digital thermometers are electronic devices. They have a sensor and a digital display. Devices with many display digits after the decimal point simulate a measurement accuracy that the device actually does not have. The displayed temperature then fluctuates in time with the measurements within seconds or fractions of a second. However, the temperature in aquaristics does not have to be measured that precisely.
The glass thermometers known in aquaristics, which are attached to the inside of the pane with a suction cup, are completely sufficient. The measuring scale has divisions of 0.1°. This accuracy is sufficient to detect temperature changes caused by water changes, a heater failure and daily temperature fluctuations, e.g. B. after turning the lights on and off. There are also such thermometers made of plastic.
Glass thermometers are robust and durable. The suction cup often has to be replaced after a year or two because it no longer sticks to the pane. However, glass thermometers should not fall on the floor. A giant pufferfish (Tetraodon mbu) has reportedly bitten through a glass thermometer. Large catfish sometimes attach themselves to a thermometer to eat algae. Especially if the thermometer is not attached to the pane and is floating freely in the water, it can hit the cover relatively hard when the catfish lets go. In such cases, it may be better to use an outdoor thermometer.
Glass thermometer Enlargement Photo: Moritz Hertel
Because glass thermometer always show slightly different temperatures, some thermometers should be compared when buying. A thermometer that reads an average should be adequate. You can also heat the thermometer to body temperature with your hand and then compare it with the display of a clinical thermometer.
A digital thermometer can be used to determine the deviation of the existing glass thermometer.
Glass or plastic thermometers used in aquaristics usually have a red filling. This is alcohol. So even if the thermometer breaks in the aquarium, there is no need to worry. The alcohol is immediately diluted in the water. However, the glass or plastic should be removed quickly so that no fish are injured.
Another filling is toluene. Toluene can harm the fish. If there is a film of hydrocarbon floating on the water surface that smells aromatic to pungent, the film should be vacuumed off immediately.
Inside the thermometer are small silver balls made of lead. These are needed as a weight so that the cap stays down and the thermometer floats upright. Lead is toxic to fish and invertebrates. The solubility of lead in water depends on the pH. Lead dissolves more easily in acidic and soft water than in hard and alkaline water. To be on the safe side, the lead balls should be removed from the aquarium as quickly as possible.
Even if there were mercury in the thermometer, it would not evaporate or be dissolved under water. It sinks to the bottom and can be easily vacuumed off with a hose. Because mercury is also toxic under water, this should be done quickly.