Phosphates in pool water: do you need to remove them?
Phosphates are a nutrient for algae, so it's tempting to treat them as the cause of a green pool. In practice, algae grows when chlorine is too low for the pool's stabilizer level. Phosphates matter at the margins. Chlorine is the main line of defense.
Where phosphates come from
- Fill water, especially from some wells and municipal supplies.
- Lawn and garden fertilizer carried in by runoff, wind or sprinklers.
- Leaves, pollen, soil and other debris.
- Swimmers: sweat, lotions and other body products.
- Some pool products, notably phosphate-based metal sequestrants and stain treatments.
Pool phosphate tests usually report in parts per billion (ppb), not parts per million. 1,000 ppb equals 1 ppm.
Why chlorine matters more
Algae needs light, warmth and nutrients, and pool water almost always has enough of all three to grow something. What stops it is chlorine. If free chlorine stays at or above about 7.5% of your stabilizer (CYA) level, algae can't get established, whatever the phosphate reading.
Example: with 50 ppm of stabilizer, that guideline puts the minimum at about 3.8 ppm free chlorine. A pool that drifts down to 1–2 ppm at that CYA level can grow algae even with low phosphates. The chlorine and stabilizer guide has the full table, and the chlorine calculator gives the dose to get there.
So if you keep fighting algae, check chlorine against CYA first, then check that the pump and filter run long enough. Phosphates come after those.
When a phosphate remover can help
There's no official standard for phosphate levels in residential pools. Remover makers and pool retailers suggest action levels ranging from a few hundred to 500 ppb or more. A remover can be worth it when:
- Phosphates are very high, for example after a flood, heavy fertilizer runoff or a pool opening with lots of debris.
- Chlorine is used up quickly and you've already ruled out low CYA, algae and a heavy swimmer load.
- You're about to close the pool for winter or go away, when chlorine can't be topped up regularly.
How to use a phosphate remover
- Clear any algae first. Shock and hold until the water is clear and passes the overnight chlorine loss test. See how to shock a pool.
- Test phosphates so you can dose for your level and pool size. Follow the product label: doses vary by brand.
- Add it with the pump running. Most removers are based on lanthanum compounds, which bind phosphate into a solid that the filter catches.
- Expect some cloudiness. The water may turn hazy for a day or so as the solid forms. Keep the filter running.
- Clean the filter afterward. The captured phosphate raises filter pressure. Backwash or clean as the label and your filter's manual describe.
Don't use a phosphate-based metal sequestrant at the same time: you'd be adding phosphate while trying to remove it.
Keeping phosphates down
- Skim leaves and debris before they break down.
- Keep fertilizer and lawn runoff away from the pool.
- Ask swimmers to rinse off before getting in.
- Check labels of stain and metal treatments for phosphate-based ingredients.
Frequently asked questions
Can phosphates make my pool cloudy?
Not directly. Cloudiness linked to phosphates is usually early algae, which comes back to chlorine level. See why is my pool cloudy.
Do phosphates use up chlorine?
No, phosphate doesn't react with chlorine. Algae and organic debris do. If chlorine disappears overnight, test for algae and combined chlorine.
Is it safe to swim after adding a phosphate remover?
Follow the product label. Many allow swimming once the product has circulated, but the water should be clear enough to see the bottom.