Metal staining is one of the most frustrating — and preventable — problems a pool can develop. A plaster pool that looked fine in the spring can show ugly rust streaks, green patches, or purple-black discoloration by midsummer. Understanding the chemistry of metal staining and how sequestrant products prevent it is essential knowledge for any pool service professional.
Three metals account for nearly all pool staining problems:
Iron enters pool water primarily from well water fill (well water in many regions contains 0.5–3.0 ppm iron or more), corroding iron or steel fittings, and some sand filter sand media. When dissolved iron (Fe²⁺) contacts chlorine or oxygen, it oxidizes to insoluble iron oxide (rust) — Fe³⁺ — which precipitates onto pool surfaces as brown, orange, or yellow staining.
Copper enters from copper heater heat exchangers (corroded by aggressive low-CH or low-pH water), copper pipe plumbing, copper ionizers, and copper sulfate algaecide. Copper precipitation produces green, teal, or black staining — often confused with algae by homeowners. Copper staining intensifies sharply when pH drops below 7.0.
Manganese is common in well water regions. At low concentrations, it's invisible. After chlorine exposure and oxidation, manganese precipitates as manganese oxide — producing dark purple, brown, or black staining that looks like mold or black algae. It is commonly confused with both and often mistreated as a result.
Sequestrants (chelating agents) bind to dissolved metal ions through coordination bonds — essentially trapping the metal in a stable, soluble complex molecule. The sequestrant molecule "cages" the metal, preventing it from reacting with chlorine or oxygen and precipitating as a stain-causing oxide.
The key distinction: sequestrants keep metals in solution. They do not remove metals from the pool. The sequestered metal complex remains dissolved until it is slowly filtered out or diluted through water replacement. This is why sequestrant treatment is ongoing, not a one-time fix — new metals continue entering the pool from fill water, and sequestrant molecules degrade over time.
| Product | Active Ingredient | Best For | Dose per 10K gal |
|---|---|---|---|
| BioGuard Pool Magnet Plus | HEDP (hydroxyethylidene diphosphonic acid) | Iron, copper, manganese | 1 qt monthly |
| Natural Chemistry Metal Free | Organic polymer blend | Iron, copper | 8–16 oz monthly |
| Orenda SC-1000 | HEDP + enzyme blend | Iron, copper, scale, LSI | 32 oz at opening; 8 oz monthly |
| Haviland Metal-Out | Citric acid + HEDP | Iron, general metals | Per label |
| Jack's Magic Blue Stuff | HEDP | Iron, copper | Per label |
HEDP (1-hydroxyethylidene-1,1-diphosphonic acid) is the gold-standard sequestrant for pool use. It is stable in chlorinated water at pool pH ranges and effective against iron, copper, and manganese. Products containing HEDP are the most reliable choice for professional use.
Adding sequestrant before or at pool opening — especially before shocking — prevents the shock treatment from oxidizing dissolved metals and causing immediate staining. Add sequestrant first, circulate for 30–60 minutes, then proceed with the opening shock and chemistry adjustment.
Calcium hypochlorite shock produces a burst of chlorine and oxidizing power. If the pool water contains dissolved iron from well water fill, this oxidation burst turns dissolved iron into rust immediately. Pre-treating with sequestrant prevents the reaction.
Any time copper sulfate algaecide is being added to a pool, run sequestrant simultaneously. The sequestrant bonds with copper and keeps it dissolved in the organic complex, preventing it from staining surfaces if pH fluctuates.
Pools filled from wells, pools in areas with copper pipes, and pools with a history of staining should receive monthly sequestrant maintenance to prevent cumulative metal buildup. Sequestrant molecules degrade — they don't last indefinitely. Monthly dosing replenishes protection.
Before spending money on stain removal, identify the metal causing the stain:
SplashLens lets you flag each account for known metal issues, log sequestrant treatments, and set reminders for monthly maintenance doses. Never miss a monthly sequestrant treatment on a high-risk account. Free and offline.
Open SplashLens Free →A metal sequestrant (chelating agent) is a chemical that binds to dissolved metal ions in pool water — primarily iron, copper, and manganese — and holds them in solution so they cannot precipitate onto pool surfaces and cause staining. Common products include BioGuard Pool Magnet Plus, Natural Chemistry Metal Free, and Orenda SC-1000.
No. Metal sequestrants prevent metal staining by keeping metals dissolved in solution. They cannot remove stains that have already formed. To remove existing metal stains, use an ascorbic acid treatment for iron stains, or professional stain removal compounds for copper and manganese. Sequestrants are a prevention tool.
Add metal sequestrant at pool opening, whenever adding copper-based algaecide treatment, after shocking with cal-hypo (which can cause iron oxidation), and as a monthly maintenance dose in pools with known metal issues or well water fill.
Green staining in pools is most commonly caused by copper precipitation. Sources include copper sulfate algaecide, copper pipe corrosion in soft/acidic water, copper-containing heater components, or ionizer systems. The staining intensifies when pH drops below 7.0.