A sequestering agent is a chemical that binds with the metals in the water to keep them in suspension so that they don't precipitate out and land on the surface of the pool making an ugly stain. There are many on the market - Jack's Magic, ProTeam, Metal Free, look for something that says it controls metals in the water, and it should say it is a sequestering agent. These are a few that I have tried, and they work. You need a sequestering agent in the water if you are getting rid of stains so that the metal that is lifted off the surface of the pool will not just precipitate out again. It is really hard to get rid of metals in the water completely, so just keeping them in suspension by having them bind with a sequestering agent is good. Let me know if you have any other questions