Water hardness

One of the most important properties of water for living organisms is its hardness. For the general aquarium and for the simple keeping of most fish, this property of water can be varied within fairly wide limits without harm. But as you get deeper into aquaristics without knowledge of the basic physical and chemical properties of water and without the ability to determine and change them – can not do without.

The simplest example is that hard water will not produce offspring from the common neon. It should be borne in mind that most fish and the same neon feel better most of their lives in harder water than they need for spawning. As for plants, they can not exist for a long time in soft water. After all, hardness is determined by the content of mineral salts in the water, which plants need as nutrition.

Here we are at the point of determining the hardness of water. Uh-huh.

Let’s understand the concept of water hardness

The hardness of water is the quantitative content of alkaline earth metal salts, mainly calcium and magnesium, in water. Therefore, calcium and magnesium salts are sometimes also called hardness salts. The more of these salts are contained in a unit volume of water (e.g. 1 liter), the harder it is.

A distinction is made between temporary, permanent and total stiffness.

Temporary stiffness

is mainly caused by calcium and magnesium hydrocarbons. This type of hardness is also called carbonate hardness. Hydrocarbonates are easily decomposed by boiling water. Water is thus freed from these salts and the well-known scale is formed on the walls of the vessel in which it was boiled.

Permanent rigidity

is caused by the content of sulfates, phosphates, calcium and magnesium chlorides in water that are not removed by boiling. This type of hardness is sometimes called non-carbonate hardness. Non-carbonate hardness can only be reduced by using an ion exchanger.

Total water hardness

The sum of the temporary and permanent stiffness values gives the value of the total stiffness.

Temporary stiffness + Permanent stiffness = Total stiffness.

Stiffness representation

To numerically express water hardness, the content of calcium and magnesium cations in water is determined. In the SI, the recommended unit for measuring hardness is mol per meter cubic meter (mol/m3). However, in practice, milligrams equivalent per liter (mg-eq/L) and degrees of hardness are more commonly used.

1 mg-eq/L = 20.04 milligrams of Ca2+ or 12.16 milligrams of Mg2+ in 1 liter of water.

Of course, it would be nice if everyone adhered to the SI guidelines. Then there would be no misunderstandings and confusion. Everyone from school would be used to the metric system of measures and would not know grief, but for various reasons it is not so. In England, it’s tradition. In Germany – habits. And in technological processes it is often more convenient to use degrees of rigidity.

But it’s not all smooth sailing with degrees of hardness either. Many countries have their own degrees of rigidity. And it is desirable to take this into account when reading books published in different countries.

How about the definition of an English degree of hardness? °e = 1 gram per 1 English gallon of water (0.2848).

I don’t know why, but I like the German water hardness degree better. Maybe I’m just used to it. So, let’s try to systematize our knowledge of water hardness units in different countries in relation to our hobby.

1°dH = 10mg/L calcium oxide (CaO).

Several tables to help you determine water hardness

In Hans J. Myland’s book there is an excellent table of the correspondence of the different degrees of rigidity. I hope it will help you when reading books from different countries as it has helped me many times.

Degree of rigidityDesignation
Designation of degrees of rigidity in different countries

Degree of rigidity conversion table

There are also specialized calculators on the Internet for converting one degree of hardness to another.

In the same book there is a table of correspondence of numerical values of rigidity to their verbal description. This representation of stiffness was accepted by aquarists of the USSR and Germany for a long time and can also be useful when reading the literature of the middle and end of the last century.

Very soft waterFrom 0 before 4°dH
Soft waterFrom 5 before 8°dH
Medium hard waterFrom 9 before 12°dH
Pretty hard waterFrom 13 before 18°dH
Hard waterFrom 19 before 30°dH
Very hard waterMore 30°dH
Word correspondence of water hardness in degrees

I hope that we have understood the concept of water hardness and its various units of measurement. If you still have any questions – ask me, I will try to answer.

Hardness of tap water

In conclusion of this article I would like to note that ordinary tap water, as a rule, has a medium hardness (9 to 12 °dH). And for the maintenance of most fish this is normal hardness values. Although in each individual locality can be quite large fluctuations.

мягкая вода
мягкая вода

You can find out about the characteristics of tap water at your local water distribution station. However, it is not certain that at any given time all the indicators of tap water will correspond to the declared ones. As for well water, in this case you can’t count on anyone but yourself. So you need to be able to determine and correct the hardness of water yourself. Read “How to determine water hardness” and “How to change water hardness”.

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