Many aquarists, such as fish keepers and shrimp breeders, use so-called conductivity meters. In this article we explain what these products can do and which measurements make sense for the aquarium hobby.

Definition

A conductivity meter is used to determine the electrical conductivity of a liquid. This value is recorded in the unit microsiemens (µS/cm). In colloquial aquarium hobby language, the terms "conductance" and "conductance meter" are also often used here. Strictly speaking, this is not quite correct, because there is a difference between conductance (the reciprocal of a resistance) and electrical conductivity (a physical quantity that indicates how well electric current is conducted).

Substances dissolved in water influence the conductivity of water. Just as certain solids (such as copper) can conduct electricity well or poorly, the number and type of ions dissolved in the water also influence the conductivity of the aqueous solution. A liquid with a low conductivity value is poor in dissolved ions, while a high conductivity value indicates that there are many ions in the water, which consequently conduct electricity better. It is important to understand that the conductivity in aquarium water refers to the sum of all ions dissolved in it. These can be elements that are also responsible for the hardness of the water (such as calcium and magnesium), but also plant nutrients such as potassium, nitrate, iron and other trace elements.

How does the conductivity meter work?

For the aquarium hobby, small, handy meters are available that are battery-operated and therefore mobile. At one end there are two electrodes under a protective cap. After removing the cap and switching on the device, you hold this end under water. The electrodes are now supplied with current, and the conductivity is determined based on the resistance of the water. A measured value can be read off the display immediately. In itself, the procedure is therefore very simple and uncomplicated to use.

Using a conductivity meter to determine water hardness

Especially in the breeding of soft-water fish and dwarf shrimp, the conductivity meter is often used to monitor water hardness. If you start with completely demineralized water, it ideally has a conductivity of 0 µS/cm. How low in ions the ultra-pure water from a reverse osmosis system is can therefore be checked very well with a conductivity meter. To supply the water with minerals, certain hardening salts are usually used. The description of the salts sometimes already mentions that a certain dose reaches a certain conductivity in µS/cm, which is then equivalent to a certain degree of hardness. In water treatment, conductivity can thus be used well to harden the RO water to a specific GH value. In the aquarium, too, as long as there is no major input of ions in another form (for example through liquid fertilizer), a conductivity measurement can sometimes allow conclusions about the general hardness. This assumption is not really correct, however, because conductivity includes all dissolved ions. There are quite a few aquarists who therefore equate conductivity and water hardness. Unfortunately, that is factually wrong. While conductivity and general hardness can well be treated as directly related values in pure water treatment with only demineralized water and the hardening salt, this is rather less the case in the practical operation of an aquarium. There are simply too many interfering factors in the form of other ions present to derive general hardness from conductivity or vice versa. Nevertheless, a conductivity meter is quite usable for other purposes, which we present below.

Does measuring conductivity make sense in a planted aquarium?

At least for determining the water hardness of the aquarium water, it does not really make sense in a planted aquarium that is also well supplied with nutrients. This is because the conductivity meter records all ions dissolved in the water and not only those responsible for general hardness. This also includes elements that enter the water through liquid fertilizer, for example. Consequently, fertilizing the plants increases the conductivity, but does not necessarily affect the water hardness, nor can conclusions about the general hardness be drawn from the increased conductivity. A high conductivity value simply indicates that there are many dissolved ions in the water.

Nevertheless, you can use a conductivity meter well to monitor the overall condition and especially the nutrient situation (see next chapter). In a planted aquarium, a comparatively sharp drop in conductivity can indicate a nutrient deficiency, which then needs to be investigated more closely.

Other uses of a conductivity meter in the aquarium hobby

What you can do, however, is use conductivity to monitor the overall condition of the water, provided you also take regular measurements. Since substances accumulate in the aquarium over the interval until the next water change, the conductivity also rises slowly and steadily during this time. Consequently, you can also interpret the level of conductivity in such a way that from a certain value on, a water change should be carried out.

Provided you monitor it regularly, conductivity can also warn you with certain outliers that something is wrong with the aquarium. If the conductivity drops or rises sharply within a short time, the water parameters seem to be changing drastically at short notice. This should give the aquarist reason to take a closer look at the aquarium, check certain things and examine the water parameters more precisely using drop tests. Here, a conductivity measurement can be used as a kind of early warning system.

Conclusion

In aquarium practice, conductivity meters are suitable within the scope of water treatment for checking ultra-pure water and for monitoring the build-up with a hardening salt. In a running aquarium, you should not simply equate conductivity with general hardness alone, since other ions also influence the electrical conductivity.

Furthermore, with regular use, conductivity meters are a good aid for indicating the need for a water change, or for detecting sudden fluctuations in the water parameters in the sense of an early warning system.