Water Hardness

Natural fresh water carries a host of disolved substances, including both organic and in-organic componds that are picked up from the soil and rocks over which the water flows over and then enters as "run off " from the land in to lakes and rivers.
The mains water that we use to fill our ponds is neither sterile or distilled, and it retains the natural mixture of dissolved componds so essential for life. There are also additional componds that all water companies add such as chlorides, sulphates,carbonates, bicarbonates, magnesium, sodium and pottasium. These elements are litterally trace elements which very few contribute to 95% of the total concentration.
The characteristics these elements contribute to water are its hardness and salinity. The salinity being the level of dissolved salt in fresh water, which is relatively low when compared to sea water for example. The hardness of water varies considerably and depends mainly on the concentrations of calcium and magnesium salts. In terms of water hardness koi can cope with a wide variation, although it is said hard water from chalky regions are beneficial as the higher levels of calcium componds it contains helps to bind up toxic metal componds such as lead.
Water hardeness has two important influences on aquatic life and these are relevant to koi and sturgeon. Firstly it influences the osmoreregulation system, in practice, hard water reduces the work load on the koi's osmoreregulation system, while koi living in soft water must have a more efficient osmoreregulation system to maintain their internal salt/water balance.
Secondly, water hardenss influences the alkalinity, in effect, a high degree of temporary hardness (caused by bio carbonate ions) increases the alkalinity of the water. So it is important to look at water hardness and PH value as connected parameters when testing your water.