I have a water feature in the back garden, i have had it for a few years now. you have to refill it with water so one day I filled it with pure water and within a week the stones had all turned green and were very slimey. After cleaning them I refilled with tap water and had no problems at all.
This problem was due to the pure water, why I don't know butmaybe some one has the answer.
Regards
Adam