Before anybody can venture a guess as to what they are, we need to have an idea of what has been put into the pool. When you tried scrubbing them, did you try a baking soda paste? Have you tried rubbing it with a vitamin C tablet to see if it's maybe metal staining?
If you can post a list of your water test results, along with a list of what has been added to the pool, it might help us help you a little better....
Bookmarks