Calcium Hardness is a measure of the calcium content of your water. High levels are known as "hard water" and low levels are known as "soft water". Low Calcium Hardness levels can lead to the damaging of the spa shell finish and equipment and will also exacerbate excessive foaming in spa water if detergents are introduced into the water via skin or bathing costumes. SpaGuard Balance Pak 300 will raise Calcium Hardness to an acceptable level of between 100 - 200 mg/l.