Calcium hardness is a measure of the calcium (Ca+2) ion concentration in water expressed as calcium carbonate (CaCO3). Pool water requires a certain amount of calcium. If the level of calcium is too low (soft water), the water will be corrosive and dissolve calcium and other minerals from plaster pool surfaces and metal equipment components. If the calcium level is too high (hard water, supersaturated), it can cause turbid water and deposit calcium carbonate scale on pool’s surfaces and recirculation equipment, particularly heat exchanging surfaces. The calcium hardness should be maintained between 150 and 1000 ppm as CaCO3 in swimming pools. Ideally the calcium hardness should be between 200 and 400 ppm.
By far the most common isotope of calcium in nature is 40 Ca, which makes up % of all natural calcium. It is produced in the silicon-burning process from fusion of alpha particles and is the heaviest stable nuclide with equal proton and neutron numbers; its occurrence is also supplemented slowly by the decay of primordial 40 K . Adding another alpha particle would lead to unstable 44 Ti, which quickly decays via two successive electron captures to stable 44 Ca; this makes up % of all natural calcium and is the second-most common isotope. The other four natural isotopes, 42 Ca, 43 Ca, 46 Ca, and 48 Ca, are significantly rarer, each comprising less than 1% of all natural calcium. The four lighter isotopes are mainly products of the oxygen-burning and silicon-burning processes, leaving the two heavier ones to be produced via neutron-capturing processes. 46 Ca is mostly produced in a "hot" s-process , as its formation requires a rather high neutron flux to allow short-lived 45 Ca to capture a neutron. 48 Ca is produced by electron capture in the r-process in type Ia supernovae , where high neutron excess and low enough entropy ensures its survival.  
Research has shown strong relationships between lower rates of chronic diseases and higher levels of vitamin D. The most well-known disease related to vitamin D deficiency is rickets . Rickets is the disease that was found to be cured by consuming cod liver oil in the early 20th century. Rickets causes soft, weak bones in children, resulting in skeletal deformities (bowed legs), impaired growth, bone pain , dental problems, and fragile bones. Unfortunately, rickets has still been seen in children not being fed an adequate amount of vitamin D. In adults, vitamin D deficiency can lead to osteoporosis and osteomalacia (both are conditions of weakened bones).