Thanks for your reply. I put the recommended dose of Metal Free in and I thought I had enough, but perhaps it wasn't. I will add more. Your advice sounds good, so I will try as you say. Yes, I do have a sand filter. I have already added a whole bottle of Polyquat. Should I add more? How much and when? And one thing I don't quite understand. Why did the pool turn green right after adding chorine? Just coincidence?