Metal stain

Metal stains show up as rust-colored spots, blue-green marks, or a dark tint in the water after you add chlorine. They come from metals in fill water, from corroding equipment, or from copper-based products.

Low pH can pull copper from heat exchangers and cause stains. A test strip with a metals check helps find the cause. Fix the source first. Then a stain remover or sequestrant can lift the marks. Do not shock a pool with high metals until you test, because chlorine can lock the stain into plaster.