I've used a mesh safety cover for years and this was the first time I EVER opened to green water. Usually, my water is clear but there's lots of dust on the bottom, which has to be vacuumed. The advantage of mesh is water doesn't pool on it so much, then freezing and adding extra weight.
The green water was due to rotten spring and a late opening--I clobbered it with LC and the water was blue in 24 hours.
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