What phosphate is
Phosphate is phosphorus in the form living things use: every plant, algae cell and bacterium needs it to grow. In pool water it shows up as orthophosphate, the simple form a test kit reacts with, and as condensed or organic forms that break down into orthophosphate over time. That gap matters later: a kit can read low while more is on its way.
Phosphate is not a sanitiser problem in itself, and chlorine cannot oxidise orthophosphate out of the water. It is a nutrient, and the whole argument about it is whether cutting the nutrient adds anything once chlorine is doing its job. It is not one of the seven readings that decide whether a pool is safe.
Where it comes from
| Source | How it gets in | What you control |
|---|---|---|
| Fill water | Many water utilities dose phosphate-based treatment chemicals | Test the hose water once |
| Scale and stain products | Orenda says these are “usually phosphate-based” | Read the label before adding |
| Some cal-hypo tablets | A phosphate scale inhibitor in the tablet | Product choice |
| Lawn and garden | Fertiliser carries phosphorus (EPA) and washes or blows in | Deck edge, irrigation spray |
| Leaves, pollen, soil, ash | Organic debris breaks down in the water | Skimmer baskets, a cover |
| Swimmers | Sweat, lotions, everything people bring in | Not much |
Sources as listed by Orenda Technologies and the US EPA; amounts vary too much by pool to put a number on any of them.
Does it feed algae? Two views
Yes, phosphate is algae food. The disagreement is whether that matters in a pool where chlorine is held at the level its stabiliser demands.
The sanitation-first view
Trouble Free Pool’s position is blunt: “Phosphates don’t ‘cause’ nor ‘create’ algae.” Algae takes hold when free chlorine falls too low for the cyanuric acid in the water, and it is killed when chlorine is brought back up. They report pools that stay clear above 4,000 ppb and others that grow algae far lower, and conclude there is no single good or bad level. On this view a remover is money spent on the wrong reading. Get the stabiliser right with the CYA calculator and hold chlorine with the dose calculator.
The remover view
Orenda, which makes a phosphate remover, argues that “algae growth is directly correlated to phosphates”. Every algae cell chlorine kills releases its phosphate back into the water for the next one. Their target is below 500 ppb. Even so, they say removing phosphates “will not kill algae, nor will it prevent algae”. Low phosphate only slows growth down.
Where both sides agree
A remover does not kill algae and does not replace chlorine. A green pool gets a proper SLAM first. Phosphate comes up afterwards, if it comes up at all.
Testing in ppb
Phosphate is reported in parts per billion: 1,000 ppb is 1 ppm. Taylor’s K-1106 is a low- and high-range drop kit whose colour card reads 0–1,000 ppb. LaMotte’s comparator kit reads in ppm instead, so check the unit before you compare two readings. Most pool kits measure orthophosphate only, so the result is a floor, not a total.
Test the fill water at the same time. If the hose reads as high as the pool, a remover is working against every top-up, and the only real fix is upstream.
Check an older K-1106 bottle
Taylor recalled Phosphate Reagent #1 bottles sold in the US before 7 August 2024: they lacked child-resistant packaging for a sulfuric-acid reagent. Taylor offers free replacements.
How removers work
The removers sold for pools work with lanthanum. Lanthanum ions combine with dissolved orthophosphate to form lanthanum phosphate, a solid. The water turns cloudy because the solid is suspended in it. The filter then has to catch it, or it settles and gets vacuumed out.
- La³⁺Remover goes inLanthanum ions, poured round the edge
- PO₄³⁻Meets phosphateDissolved orthophosphate in the water
- LaPO₄Turns solidA fine precipitate: the water clouds
- FilterGets caughtFilter or vacuum takes it out
That is why the filter matters more than the bottle. SeaKlear’s instructions: clean or backwash before dosing, run the filter continuously for at least 24 hours after, clean again if pressure climbs 10 psi over normal, and expect cloudiness for up to 48 hours with no swimming while it lasts. Trouble Free Pool warns that built-up lanthanum phosphate “can irreversibly clog up cartridge and DE filters”, and advises waiting 48 hours before cleaning. Plan the cartridge clean or backwash before you open the bottle, and watch the pressure gauge for the next two days.
| Phosphate reading | SeaKlear dose, 10,000 gal |
|---|---|
| 250 ppb | 1 oz |
| 1,000 ppb | 4 oz |
| 3,000 ppb | 12 oz |
| 9,000 ppb | 32 oz |
One maker’s label, printed so you can see the scale. Products differ in strength, so dose by the bottle in your hand.
When it is worth treating
- Skip it if the pool is clear and chlorine holds. A number on a kit is not a problem by itself.
- Consider it on a salt pool whose cell seems short of output, but only after ruling out salt level, cell scale, cell age, the output setting and run time. Trouble Free Pool calls the success anecdotal; the salt vs chlorine comparison covers what the cell needs first.
- Consider it at closing as insurance for a covered pool that will sit for months with falling chlorine. The winterizing chemicals guide covers the rest of that dose.
- Never as an algae treatment, and never when you cannot clean the filter in the next two days.
If a remover goes in and the water stays hazy past two days, treat it as cloudy water with a known cause: filtration, not chemistry.