If there is Staining on the liner, fibreglass, or concrete in the pool that is dark and not able to be removed by scrubbing, it may be evidence of metals in the water. This may also cause the water to turn an abnormal colour. Metals can reside in pool water because of Metal foreign objects entering the water (such as pennies or other coins), copper piping and fittings, the use of Copper Sulphate to clear algae, copper based algaecides, source water, or when water balance problems cause your heater to corrode. It is best to bring a water sample to Buds as soon as the staining or discolouration occurs so that appropriate measures can be taken as soon as possible.