Metal stain
Also called: iron stain, copper stain, rust stain.
A metal stain is dissolved iron, copper or manganese that chlorine has oxidized and plated onto the pool surface: rust-brown or orange for iron, teal or blue-green, sometimes black, for copper. Under a crushed vitamin C tablet an iron stain lightens and a copper stain goes darker or black.
Why it matters
The metal arrives three ways: fill or well water high in iron, a metal part corroding because pH ran too low, or copper algaecides used too freely. Ascorbic acid lifts an iron stain fast, but it moves the metal back into the water and does not remove it, so you add a sequestrant the moment the stain lifts and rebalance slowly, with no SLAM for at least two weeks. Ascorbic acid is for iron only; on copper it makes things worse, and the real copper fix is cutting off the source. Only diluting or replacing water actually removes metal. A mark that lightens under a trichlor tablet instead is organic.
In numbers
For an iron stain, with free chlorine at zero, about half a pound of ascorbic acid per 10,000 gallons lifts the stain within about half an hour.