COPPER, IRON, and OTHERS
Copper is one of nature's elements. It is also used in the equipment and plumbing in most swimming pools. Copper, in its free state, may also be present in cheaper algaecides.
Corrosive water (low pH, low TA, low calcium hardness) caused by improper water balance or misuse of chemicals can cause copper to be dissolved into the pool water. The water supply you use for topping up the pool may also contain high levels of copper, iron and other metals.
The maximum level for copper is 1.0ppm, though lower levels are recommended. At high concentrations, the copper deposits out of the water and onto hair, fingernails or pool walls causing green stains. High levels of copper can also cause green water.
Iron in the pool causes the water to turn brown or green. It can also cause staining of the pool walls. Iron has much the same causes as copper (i.e. corrosive water or high levels in the top-up water). The maximum level of iron is 0.3ppm although iron-free water is desirable.
Copper and iron can be "locked" into the water using chemicals (chelating agents). This prevents the metal from discoloring the water or staining the pool.
Magnesium and manganese in the water increase the hardness and can cause scale. See also calcium hardness.