A metal found in some algaecides and most “mineral systems” that can cause green, gray, or black stains on the pool surface and also occasionally tints the water a brilliant clear transparent emerald green. Copper can also get into the water if you have a copper heat exchanger (common in gas heaters) and your PH is below 7.0 for an extended period. If you have copper in the water you will often need to use a sequestrant to prevent stains from forming. Copper stains can be especially difficult to remove.