The hardness of water refers to the combination of substances based on calcium
(Ca) and magnesium (Mg) that are contained in it. The main substances, known as
salts, are carbonates, bicarbonates and sulfates.
Water with zero hardness does not contain any of these salts; this is the case
with distilled water.
The water in some areas can be particularly hard, mainly due to the presence
of limestone (or calcium carbonate).
There are kits on the market that offer even the novice
aquarist the panoply of tests required to control the majority of the main parameters
The hardness of water really depends on the land through which it has passed:
the more calcium and magnesium the rocks contain, the harder the water. The effects
of this can be seen in domestic use: a washing machine, for example, will require
more detergent. Above certain limits of hardness (see the table on page 17), water
is unfit for human consumption or any other use. Water with a low degree of hardness,
i.e. containing few calcium and magnesium salts, is considered soft. Water with
a high degree of hardness is classified as hard.