CliniPool / Understand your water
Phosphates: the hidden pantry of algae
- Phosphates are nutrients for algae: they don't cloud the water directly, but they feed it.
- Sources: dust, leaves, garden fertiliser, rain, top-up water, certain products.
- Very high phosphates = algae coming back fast, even with correct chlorine.
- Not a routine parameter for everyone — useful mainly for unexplained recurring algae.
Phosphates are your pool’s invisible guests: they don’t cloud the water, don’t colour it, can’t be smelled — but they make up the pantry of algae. They’re nutrients: as long as sanitisation holds, the chlorine kills the algae before they can enjoy the buffet, and phosphates pose no problem at all. But when the sanitiser weakens, a very high phosphate level turns the slightest window of weakness into express green water. It’s not a parameter for everyone to watch daily — it becomes precious in one specific case: algae that keep coming back despite correct chlorine. The CliniPool app knows when this parameter deserves to take the stage.
What they do (and don’t)
Let’s be clear: phosphates are not dangerous to swimmers and don’t turn the water green by themselves. Their role is indirect but real: they feed algae. Water full of phosphates is fertile ground; water low in phosphates is a desert where algae struggle to take hold. With correct sanitisation, the fertile ground stays harmless — the chlorine plays policeman. The problem arises at the junction of the two: plenty of food + a little less sanitisation = the algae explode before you see it coming.
That’s exactly what explains certain “baffling” green waters: chlorine fine on the test, but algae back every couple of weeks. Often, a saturated phosphate pantry is in the picture.
Where they come from
Phosphates build up slowly, from a thousand small sources. Leaves and plant debris breaking down, dust carried in by wind and rain, runoff from a fertilised garden toward the pool, top-up water depending on the region, and even certain treatment products or sequestrants that contain them. Season after season, the level climbs — all the faster if the pool is surrounded by vegetation or poorly shielded from outside inputs.
Should you deal with them?
The honest answer: not systematically. Dosing phosphates “on principle” on well-kept water is pointless — chlorine is more than enough. The right reflex is to measure them in cases of unexplained recurring algae: if sanitisation is correct but the green keeps coming back, checking phosphates is part of the investigation (alongside stabiliser, the other silent saboteur). The real-life case of recurring algae (“See also”) illustrates this kind of detective work.
If the level is very high, a phosphate remover precipitates them to be filtered out, alongside a serious clean of the sources (leaves, debris). But the absolute priority always stays the same: correct sanitisation first. Starving the algae is a backup, not a substitute for chlorine.
In one sentence
Phosphates are the pantry of algae: invisible, harmless in themselves, but formidable when sanitisation weakens. You don’t watch them every day — you take them off the shelf when algae recur for no apparent reason. The app knows when to ask the question, and steers the investigation instead of stacking up products.
Frequently asked questions
Do you always need to treat phosphates?
No. Well-sanitised water holds perfectly fine with phosphates present: the chlorine kills the algae before they can raid the pantry. Phosphates become a topic mainly when algae keep coming back despite correct sanitisation.
Where do phosphates in a pool come from?
From everywhere: dust and leaves, runoff from a fertilised garden, rain, top-up water, and sometimes certain treatment products or sequestrants. They build up slowly over the season.
Do phosphates turn the water green?
Not directly: they don't colour the water. But they feed algae, so a very high level combined with weakening sanitisation clearly speeds up the onset of green water. It's an aggravating factor, not the root cause.
How do you lower phosphates?
With a phosphate remover (phosphate sequestrant) that precipitates them to be filtered out, alongside a good clean of the sources (leaves, debris). But the priority always remains correct sanitisation: chlorine first, phosphates as a backup if needed.