Pool phosphates: do they matter?

What phosphates are

Phosphate is phosphorus combined with oxygen. Phosphorus is an essential nutrient for every living thing, including algae and bacteria. Pool phosphate tests measure orthophosphate, the simple dissolved form that organisms can use directly. Results are usually given in ppb (parts per billion): 1,000 ppb = 1 ppm.

Two points from PHTA's source-water fact sheet are the core of the debate:

• Phosphate is already fully oxidized, so it doesn't react with free chlorine and creates no chlorine demand by itself.
• When sanitizing is inadequate, algae and bacteria multiply using phosphate as a nutrient, and that growing population then consumes chlorine.

In other words, phosphate becomes a problem only after chlorine has already fallen behind.

Where they come from

• Tap water. Many water utilities add phosphate on purpose as a corrosion inhibitor to protect pipes and limit lead and copper release, as part of EPA-driven corrosion control. PHTA notes this means high phosphate readings can come straight from municipal water. Your utility's annual water quality report says whether it does this.
• Nature. Leaves, pollen, grass clippings, soil, bird droppings, and runoff from fertilized lawns and gardens.
• Swimmers. Sweat, lotions, and other residues add small amounts.
• Some pool products. Certain stain, scale, and cleaning products are phosphorus-based. If you add them often, ask the maker whether they add phosphate.

Because phosphate comes in with top-up water and debris all season, a remover lowers the reading only until the next source brings more.

Do they cause algae?

Algae need phosphorus, but only a little, and many outdoor pools have more than enough from rain, leaves, and fill water regardless of what you do. The deciding factor is whether chlorine kills algae faster than it can grow.

That depends on the ratio of free chlorine to stabilizer (CYA). The CDC's Model Aquatic Health Code 2024 Annex shows the active form of chlorine falls as CYA rises. Our engine sets the minimum FC at about 7.5% of CYA: 3 ppm at CYA 40 and 6 ppm at CYA 80. Pools that turn green with "normal" chlorine readings are usually below that minimum, whatever their phosphate level. See pool chlorine levels for your number.

What the other side says: makers of phosphate removers argue that very low phosphate gives you a safety margin. A lanthanum patent assigned to a pool-chemical company (US 6,338,800) itself describes algae as not growing when "appropriate levels of sanitizers are used" together with very low phosphate. Its tests measured how fast phosphate was removed in 1,100-liter tanks. They didn't test whether algae grows in a properly chlorinated pool with ordinary phosphate levels. We found no independent controlled study showing that removing phosphate prevents algae in a properly chlorinated home pool.

When a remover makes sense

Phosphate removers typically use a lanthanum salt, which binds phosphate into an insoluble particle that the filter catches (PHTA). They can be reasonable when:

• You can't keep chlorine up for a while, for example a pool left unattended for weeks, and you want an extra margin.
• You use a biguanide (PHMB) sanitizer. PHTA notes biguanide can react with phosphate and form a precipitate that raises sanitizer use.
• Your fill water is very high in phosphate and you're refilling or opening a pool with low stabilizer and chlorine lagging behind.

Expect the water to turn cloudy for a while as the particles form. The same patent notes that fast-acting lanthanum chloride makes particles fine enough to cloud the water and slip past filters. Clean or backwash the filter afterward, since the precipitate collects there. Follow the label dose for your reading and volume.

What to do instead

1. Know your CYA and the minimum free chlorine that goes with it. Keep FC at or above it every day, not only after you shock.
2. Measure chlorine loss overnight. Test FC at dusk and again before sunrise. With no sun, a healthy pool loses very little. A common forum guideline (TroubleFreePool's overnight chlorine loss test) treats a drop of more than 1 ppm as a sign that something is consuming chlorine, often early algae you can't see yet. Details: chlorine not holding.
3. Brush and vacuum walls, steps, and corners weekly. Algae start where circulation is weak.
4. If algae has started, identify it on pool algae types and shock with a pool shock product, following its label for the amount and for when you can swim again.
5. Remove debris promptly and empty baskets. Leaves add phosphate and use up chlorine.

All target levels on one page: pool chemistry chart.

FAQ

What causes phosphate in a pool?

Tap water treated with phosphate corrosion inhibitors, leaves, pollen, soil, lawn fertilizer runoff, bird droppings, swimmers, and some cleaning or stain products. Tap water and plant debris are usually the biggest sources.

What is a phosphate treatment for a pool?

A liquid product, usually based on a lanthanum salt, that binds dissolved phosphate into tiny particles for the filter to catch. It lowers the phosphate reading but doesn't sanitize. The water may turn cloudy for a while, and the filter needs cleaning afterward.

Do pool phosphates matter?

Much less than chlorine does. Phosphate doesn't consume chlorine, and algae can't grow in a pool that keeps free chlorine above the minimum for its CYA. Phosphates matter more when chlorine keeps falling short, and in biguanide pools.

Sources

• PHTA fact sheet, Source water and its effects on pool and spa maintenance (2020): phta.org (PDF)
• CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
• US Patent 6,338,800, Methods and compositions using lanthanum for removing phosphates from water: uspto.gov (PDF)
• TroubleFreePool wiki, Phosphate removers (secondary source, forum wiki): troublefreepool.com