Black spots in hot water system?

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

what type of system do you have, it could be muck out of tank in loft , copper tank. we can't help if you don't help us.

hi-spec plumbing
October 2006