I have the same issue - I am not interesetd in how to clean it - I want to know what causes it and is it dangerous. I have two header tanks in the loft above the hot water cylinder in the airing cupboard below - one the water is dirtyish and the other clean - when I ran the bath for 15 mins (hot water) to see which tank started to fill it was the one with clean water.
The spots are in the sink and the bat and when rubbed they smear like ladies mascara
PhilC
February 2010