What the guideline says
The conductance measures the electrical conductivity of the water. The electrical conductivity of water depends on the salinity of the water, since only the ions of the salts conduct electricity in the water. Ions of different salts conduct water differently.
The conductivity therefore provides information about the salts dissolved in the water. The conductivity (conductivity) is measured in µS/cm (microsiemens/cm). The salinity is calculated as follows: LF x 0.75 = total salinity in mg/l. If the carbonate hardness (KH) and the total hardness (GH) are known, it can be determined how many other salts are in the water, e.g. B. sodium, chloride and nitrate.
How conductance is measured
There are special devices for measuring the conductivity, so-called conductivity measuring devices, e.g. B. from the company Greisinger.
The importance of conductivity in aquaristics
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In aquaristics, the conductance value is used to control fully desalinated water, brackish and seawater, and to control the increase in salinity in the aquarium water. The plants consume salts, so they lower the conductivity, but feeding them increases it.
If the master value does not change, e.g. B. between a morning and an evening measurement, then the water quality has not changed much.
Indirectly, a measurement after feeding the fish can be used to determine whether the fish have eaten the food or whether the food has dissolved in the water.
Nevertheless, special measurements of e.g. B. nitrite, nitrate and phosphate can not be dispensed with. Due to the complex processes in the aquarium, no clear assignment can be made as to which substances are responsible for possible conductivity changes. Nor can it be said unequivocally that an unchanged conductivity indicates stable conditions in the aquarium. Acid ions conduct electricity much better than phosphate ions. A greater increase in the phosphate value can therefore go unnoticed if the pH value changes slightly at the same time. The difference from 0 to 5 mg/liter nitrite results in a difference of only 12 µS/cm.
A conversion from nitrite to nitrate does not result in any change in conductance. Although changes are much more noticeable in very soft water due to the very small conductivity, some effects on conductivity are also very small. So it is practically imperceptible if any remaining carbonate hardness of 0.5° dKH is destroyed by nitrification.
The following lists show how few concrete clues the conductance measurement gives.
Conductance can be increased by:
- feeding
- fish excretions
- nitrification
- Change in pH
- temperature change
- water conditioner
- fertilizer
- Agents such as pH-Minus and pH-Plus
- peat filtering
- Clouds in front of the window of a sunny aquarium
Conductance can be reduced by:
- Some ideally dosed water conditioners
- temperature change
- Vigorously growing aquatic plants
- water change
- denitrification
- peat filtering
- Clouds in front of the window of a sunny aquarium
Does the conductance indicate the need for a water change?
The sum of all salts is always measured. It cannot be determined whether z. B. the amount of chloride has increased significantly, the amount of manganese, potassium, iron decreases significantly and at the same time nothing changes in the nitrate and phosphate values.
Different waters can have the same conductance and still have a completely different composition. In this way, fresh tap water can have the same conductance as aquarium water that has not been changed for weeks. Therefore changes in conductance can indicate that a water change is necessary; however, the opposite cannot be concluded. If the conductance remains stable, it does not mean that a water change is not necessary.
When does it make sense to measure the conductance?
Fish control the osmotic pressure in their bodies with sodium and chloride ions via what are known as chloride cells in their gills. In the case of soft-water fish, which are used to low conductance values, adaptation to different sodium and chloride concentrations in the water is only possible to a limited extent. The adaptability of different fish species is different. The concentration of the salts is therefore very important for soft-water fish and monitoring with the help of conductivity measurement makes sense. The same applies to monitoring the spawning of sensitive fish species. Since water always flows in the direction of lower salt concentrations according to the principle of osmosis, the fish spawn can be damaged if the salt concentration in the egg differs too much from the salt concentration in the water.
The relationship between conductance and hardness
The more salts are dissolved in the water, the more the water conducts electricity, depending on the temperature. The conductivity meter is used to determine how well water conducts electricity. It can only be measured how well all conductive minerals in the water conduct together. The conductivity of a single substance cannot be determined. The total hardness therefore determines the conductance. The conductance measurement can thus be used to determine whether the salt content of the water is increasing. In oxbow lakes z. B. can be determined whether and to what extent the water is salted over time. Through the feed and the refilling of evaporated water, new substances are constantly entering the aquarium, which can only be removed from the aquarium again by cutting the plants. The water should be changed at the latest after a sharp increase in the conductivity in order to remove excess substances from the aquarium.
Basically, water with a lower conductivity has a lower total hardness. Water with a higher conductivity is correspondingly harder.
However, it is not possible to use an exact formula to calculate the hardness from the conductivity or vice versa. According to a rule of thumb, water with 1° dH generates at least a conductivity of 35 µS/cm or more. This rule only applies to tap, ground and river water. The rule of thumb does not apply to old, brackish and lake water. The rule does not apply to carbonate hardness because carbonate hardness is a part of total hardness. The rule is applicable, e.g. B. in soft water, measure too roughly for the drop test.
Because the content of sodium, chloride and sulphate in different types of tap water can vary greatly, the conductance can be pushed up considerably. There exist e.g. B. Tap water with a total hardness of 17° and a conductance of over 700 µS/cm. On the other hand, there is tap water with a total hardness of 16.5° and a conductance of 560 µS/cm.
The rule of thumb can therefore only be used to a limited extent for tap water. However, it is helpful to monitor the dilution of a certain tap water with osmosis water using the conductivity meter if the relationship between the conductivity and the total hardness of this tap water is known. Water analyzes can also be roughly checked for accuracy by measuring the conductance.
The relationship between conductance and salinity
According to another rule of thumb, 1 per mil salt produces a conductivity of approx. 1600 µS/cm.
For accurate measurements and derivations, the temperature coefficient and non-linearity of the function must be considered.
Conductivity measuring devices with a suitable measuring range are very well suited when brackish water is to be produced and controlled.