Sequestrant is any of several chemicals that bind to metal ions dissolved in the water and prevent them from depositing as stains. All sequestrants break down slowly over time, so you need to add more at regular intervals to replace the portion that has broken down.[1]
There is no point in using sequestrants day to day unless you know you have metal problems. The only time sequestrant is worth using as a general protection is when fresh plaster is applied, as metal stains on brand new plaster can never be removed.
Sequestrants based on HEDP, phosphonic acid, or phosphonic acid derivatives are the most effective. ProTeam's Metal Magic and Jack's Magic The Pink Stuff (regular), The Blue Stuff (fresh plaster), and The Purple Stuff (salt) are some of the top sequestrants. You can also find many other brands with similar active ingredients, some of which are noticeably less expensive.
