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Iron present in sufficient quantities in pool water can cause discoloration. It can stain pool surfaces as well as hair and nails. Iron will usually will turn the pool darkish brown but sometimes it will show a green hue.
Iron can be introduced into the pool from fill water as well as from direct contact with metal components exposed to corrosive conditions. Cast iron headers from a gas heater can be one way iron is introduced into a pool.
To treat the problem you can either add shock to your water or introduce a sequestering or chelating product.
The desired iron level should be 0 ppm