Phosphates in Pool Water: When to Worry
By The Aqualitx team at STEMania · Updated
Phosphates are a nutrient for algae. They arrive with leaves, pollen, lawn fertilizer, rain, some source water, and certain pool chemicals. Phosphates aren't harmful to swimmers and don't cloud the water on their own — they just make algae easier to grow and harder to get rid of.
Levels worth acting on
- Under 200 ppb: nothing to do.
- 200–500 ppb: elevated. Worth reducing if you've had algae trouble or chlorine that disappears fast.
- Above 500 ppb: high. Reduce it, especially in warm weather.
Note the unit: phosphate tests read in ppb (parts per billion), not ppm. 1,000 ppb is 1 ppm.
Chlorine comes first
A pool with free chlorine at 7.5% of its CYA or higher won't grow algae no matter how much phosphate is in it. Phosphate remover is a second line of defense — useful when chlorine is hard to hold steady, or as the follow-up after killing an algae bloom so it doesn't return. It's not a substitute for proper chlorine.
How much phosphate remover to use
- Lanthanum-based removers (e.g. Orenda PR-10000) are dosed to the reading: about 1 fl oz per 10,000 gallons for every 1,000 ppb. 800 ppb in 20,000 gallons is 0.8 × 2 ≈ 1.6 fl oz. Follow your product's label — concentrations vary.
- Combo clarifier + phosphate binders use a fixed dose, typically about 8 fl oz per 10,000 gallons, repeated weekly. Easier, but they pull phosphate down more slowly.
How to add it
- Run the pump, and pour the remover slowly in front of a return jet.
- Expect the water to go cloudy for a day — that's the phosphate being bound into particles the filter can catch.
- Clean or backwash the filter within 24–48 hours to remove what it caught.
- Re-test phosphates after 10–14 days, not the next day.
Keeping phosphates down
- Skim leaves and debris before they break down in the water.
- Keep fertilizer and sprinkler runoff out of the pool.
- Check your source water — some municipal water carries phosphates.