Sequestrant

Sequestrants chelate, or chemically grab hold of, dissolved metals like iron, copper, and manganese that enter pool water from fill water, corroding equipment, or certain algaecides, keeping them in a soluble form so they can't oxidize and precipitate onto plaster or liner as rust-colored or blue-green stains. They're typically dosed after a fill-water test shows metals present, and again periodically afterward, since sequestrants gradually get consumed and need topping up. They treat the metal problem, not the sanitizer or pH; a pool can have flawless chlorine and pH levels and still stain if metals go unsequestered.