Sequestrant
Also called: chelating agent, metal control, stain & scale control.
A sequestrant, also called a chelating agent, binds dissolved metal such as iron so the metal stays in the pool water instead of plating out as a stain. A sequestrant does not remove metal from the pool, and it does not lift a stain that is already on the wall.
Why it matters
It is the step people skip after an ascorbic acid treatment. The acid moves iron off the wall and into the water, and the moment chlorine comes back up that iron re-plates unless a sequestrant is holding it. It is maintenance, not a cure: sequestrant breaks down over time, so you re-dose on a schedule. The effective ones are HEDP or phosphonic-acid based. Most are phosphate-based and raise your phosphates; Orenda makes a phosphate-free chelating alternative. Only diluting with metal-free water actually removes the metal. Against calcium scale, balanced water beats the bottle.
In numbers
Filling from a well? Add a startup dose of sequestrant when you fill, about 1.5× the normal dose for well water, so the chlorine doesn't oxidize the iron into a fresh stain.