How do metals get into pool water?

Metals are naturally occurring substances found everywhere, including stone, soil and water

Rainwater is acidic and tends to dissolve the metals found in soil and stone, thereby increasing metal concentrations in surface and ground water.  Furthermore, because all municipal and well water contains some metal, introducing metals into your pool is unavoidable.

Marble, flagstone and granite, for example, naturally contain iron, which can leach into a pool after every rain. Salt used in all salt water systems also contains iron. Copper can be introduced into pool water from the heat exchanger found in the pool heater, from copper-based algaecides, or from poorly maintained ionization systems. Manganese and nickel, also commonly found in pool water, are frequently in the source water used to fill pools.

Although you may not see the metals (unseen metals) they are present and will cause discoloration or staining as they get more concentrated with evaporation.