Hardness is a term which relates to the amount of dissolved minerals present in the water. Water with a high concentration of dissolved minerals is referred to as ‘hard’, water with low concentrations of these minerals is ‘soft’.Hardness is usually measured in °dH, a German measurement in which each °dH is equivalent to 17.9 mg/l calcium carbonate.

There are two types of hardness;

General Hardness (GH), or Total Hardness, is a measure of calcium and magnesium salts. These salts are vital for life in the pond as they directly affect the cell functions in plants, fish and microorganisms. GH should be between 4° and 10°dH.

Carbonate Hardness (KH), or Temporary Hardness, is largely calcium carbonate. KH is commonly seen as the build up of calc, or fir, which collects on kettle filaments and around taps. In a pond the KH acts as a buffer which prevents sudden pH changes. KH should measure between 3° and 10°dH.

If the GH or KH readings are too high then rain water should be added to dilute the water’s hardness. If the readings are too low then there are dissolved minerals available as pond additives which can be used.