Is It Metals or Algae? How to Tell the Difference
The first step is the most important: correctly identifying the cause. The two most common causes of brown pool water — metals (iron or manganese) and algae — require completely different treatments. Using the wrong approach can permanently stain your pool or waste days of effort.
🌿 Signs It's Algae
- Brown cloud forms when wall is brushed
- Pool turned brown after poor maintenance period
- Slimy or slippery feel on surfaces
- Problem worsened gradually over days
- Mustard/yellowish-brown tint to water
- Vitamin C test has no effect on stain
Iron in Pool Water
Iron is the most common metal contamination in residential pools. It enters the pool primarily through:
- Well water: Groundwater naturally contains dissolved iron; many wells have iron concentrations of 0.5–5+ ppm
- Corroding pipes or equipment: Old iron pipes, rusty pump components, or deteriorating metal fittings leach iron over time
- Heavy rain runoff: Can introduce iron-rich soil and organic matter from the surrounding ground
Dissolved iron in pool water is invisible. The water may look crystal clear with iron present. The problem occurs when you add chlorine — chlorine oxidizes dissolved iron (Fe²⁺) into iron oxide particles (Fe³⁺), which is essentially rust. The reaction is immediate and dramatic: clear water turns orange or dark brown within minutes of adding shock.
Manganese in Pool Water
Manganese is less common than iron but behaves similarly. It typically enters pools through well water or municipal water supplies in areas with naturally high mineral content. The key difference is the color:
| Metal | Water Color | Stain Color | Common Source |
|---|---|---|---|
| Iron (Fe) | Orange / Rust-brown | Orange / Red-brown | Well water, old pipes |
| Manganese (Mn) | Purple / Black-brown | Purple / Dark gray | Well water, municipal supply |
| Copper (Cu) | Blue-green tint | Blue-green / Teal | Copper pipes, algaecide |
The fix for manganese is identical to iron: metal sequestrant, lower pH, extended filtration. Both are removed the same way — they just produce different colors when oxidized.
Fixing Metal Contamination — Step by Step
This process works for both iron and manganese. Follow it in order — skipping steps will cause the treatment to fail or leave permanent staining.
Lower pH to 7.2
Add muriatic acid to bring pH to exactly 7.2. At lower pH, metals are more soluble and sequestrant products work more effectively. Do not go below 7.0 — it becomes corrosive to surfaces and equipment.
Add Metal Sequestrant — Not Clarifier
This is the critical product. Metal sequestrant (chelating agent) binds to dissolved metal ions and keeps them in suspension so the filter can remove them. Add per the product label for your pool volume. Popular products: Metal Magic, Jack's Magic Blue Stuff, Proteam Metal Magic.
Run Filter Continuously for 24–48 Hours
The sequestrant holds metals in solution; your filter removes them. Run pump at full speed. Backwash or clean filter every 8–12 hours — it will load up with metal particles quickly.
Vacuum to Waste
Any settled metal debris on the floor should be vacuumed directly to waste — not through the filter. This prevents recirculating the particles and clogging the filter unnecessarily.
Rebalance Chemistry and Shock
Once water is clear and metals are removed (test with a metal test kit if uncertain), adjust pH back to 7.4–7.6, then shock normally to restore proper sanitation levels.
Maintain with Monthly Sequestrant Dose
Metal sequestrant breaks down over time (especially with chlorine and UV). A monthly maintenance dose prevents metals from re-oxidizing and re-staining. Budget ~$10–15/month for this.
Preventing Metal Issues
Prevention is far easier than treatment. These habits eliminate most metal problems before they start:
Pre-treat Fill Water
When filling from a well, run the water through a sediment filter and add sequestrant to the pool before adding chlorine. Never shock immediately after filling well water.
Test Source Water
Have your well water tested for iron, manganese, and copper annually. Knowing your baseline helps you dose sequestrant correctly and avoid surprises.
Monthly Sequestrant
Add a maintenance dose of metal sequestrant monthly during swim season. This keeps any metals that enter the water bound and filterable before they stain.
What If It's Algae After All?
Some algae types can look brown or yellowish rather than the classic green:
- Mustard algae (yellow-green algae): The most common brown-look algae; often appears as a dusty, yellowish-brown coating on shaded surfaces (walls, steps); brushes off easily but returns quickly; chlorine-resistant; requires specialized mustard algae treatment (super-chlorination + algaecide)
- Green algae in early stages: Very early-stage green algae can appear more olive or brownish before it proliferates into the recognizable bright green
- Organic debris tannins: Heavy leaf fall can leach tannins into pool water, creating a brown or tea-colored tint; this is organic matter, not algae or metals — treated with shock and extended filtration