Most rainwater stored in water tanks has a ph of approximately 4.5 and as PH neutral is 7 on the scale the water can be a little acid this can be the cause of corrosion to copper pipes as the copper corrode from the inside of the pipes it settles on the tiles in your shower, bathtub or Basin the blue green stain can be removed with a salt and white vinegar mix concrete tanks can help to reduce the problem by alkaline in the water from the lime leaching from the concrete however this only happens for a short period in the early life of the tank the slime on the walls and the age of the Tank reduce the amount of lime that's available to alkaline the water some people balance the water to a neutral pH with lime chips in a meh bag hanging in the water tank. A more efficient method is to install a calcite filter that alkalizes Water to the pH required. These need to be changed approximately every 2 years. Copper in high concentration is toxic to humans.The increase in acidity of rainfall and the use of poly tanks is said to be causing this problem to be more prevalent.
To clean up copper stains on bathroom basin and tubs. Combine one cup of white vinegar with 1 tbsp of salt and boil until the salt dissolves. Soak and rub with towelling cloth or a string mop to remove blue green stain.