The joint hardness of the water is determined by calcium and magnesium. This is called the GH value of the water and it is expressed in degrees of German hardness (°GH). A proper GH value for pond water ranges between 8°and 12° GH. This is important for several reasons. Most species of pond plants will optimally grow within these values, furthermore the development and the activity of micro-organisms in this medium hard water will be optimum too.
The supply of oxygen at these values is safeguarded best. If the supply of CO2 will increase outside the season, the chance of acidification is remote. At GH values of 8° to 12° GH the surplus of CO2 will be bound by calcium and consequently there is no danger for acidification or lack of oxygen.
Owing to weather influences (rain, hail, snow) and biological processes the pond water will be softened continuously. It is necessary therefore to check the GH value a number of times a year. At any rate in spring and in autumn. If the value should be less than 8° GH, it is important to increase it.