Phosphates don't visibly dirty the water and aren't directly harmful to swimmers at the concentrations found in a pool. The issue is something else: they're the algae's favorite food, and as long as they're present in any quantity, a temporary dip in chlorine, a weekend with more swimmers, a couple of days without a check, a summer storm, can turn into an algae outbreak much faster than expected.
Phosphates: food for algae
Where phosphates come from
You don't need to add them on purpose: they arrive on their own, constantly, through many different routes.
- Leaves, pollen and plant debris that fall into the water or onto the surrounding lawn and end up washed into the pool.
- Sunscreen and cosmetics from swimmers themselves, one of the steadiest sources in pools with heavy family use.
- Garden fertilizers, especially after rain or watering, when runoff carries residue toward the pool.
- Tap water used to top up the pool, which in some areas already carries a low baseline load of phosphates.
- Some maintenance products, certain flocculants and clarifiers contain phosphate-based compounds, so they can paradoxically add a little phosphate with every use.
The result is that phosphate tends to build up over time, season after season, if nothing is ever done to specifically reduce it.
Why they're a problem even when chlorine looks fine
Chlorine and phosphates work on different levels. Chlorine kills off algae and microorganisms already in the water; phosphates, on the other hand, are the nutrient that lets any algae that survives, and a small amount always survives, multiply rapidly the moment it gets the slightest opening. A pool with high phosphates can keep chlorine in its correct range for weeks with no apparent issue, and still "explode" into algae the moment chlorine dips slightly, for instance after several days of intense sun and heavy bather load. That's why it's worth tracking as a parameter in its own right, alongside pH, alkalinity or calcium hardness, which any routine water test already covers.
How they're removed
The usual way to lower phosphate is with phosphate-removing products, which work on a simple principle: they contain compounds (typically lanthanum salts or similar agents) that react with the dissolved phosphate in the water and convert it into a solid, insoluble precipitate that's no longer available to algae. That precipitate stays suspended as a fine haze and gets pulled out by the filter; in pools with a lot of built-up phosphate it can help to run a flocculant first so the filter catches it more efficiently. This process doesn't replace chlorine or the rest of the usual treatment, it complements it, and it's normally only needed occasionally, not every week.
How to stop them building up again
- Cover the pool when it's not in use, especially during leaf-fall or high-pollen season.
- Shower before swimming, it cuts down the amount of cosmetics and sunscreen that end up in the water, a simple habit with more impact than it seems.
- Avoid fertilizing the lawn or nearby plants right before predicted rain, or do it well away from the pool basin.
- Test phosphate levels occasionally, not only once an algae problem has already appeared, it's measured with a specific kit, different from the one used for chlorine and pH.
Frequently asked questions
Are phosphates harmful if I swim with them in the water?
They aren't toxic to people at the concentrations seen in a pool. The issue isn't swimmer health, it's that they feed algae and force the chlorine to work much harder than it should have to.
Why do I have phosphates if I've never added them to the water?
You don't need to add them on purpose. They arrive on their own with leaves and pollen, with swimmers' sunscreen and cosmetics, with garden runoff after rain, and in some areas even with the tap water used to refill the pool.
Is it enough to raise the chlorine if my phosphates are high?
That can mask the symptom for a while, but it doesn't fix the cause. As long as the phosphate is still there, any temporary dip in chlorine can trigger another algae outbreak. Reducing phosphate at the source is what actually works.