Phosphates are the algae food source most pool owners ignore until their water won’t stay clear no matter how much chlorine they add. Understanding what phosphates are, where they come from, and how to remove them turns an unsolvable algae problem into a 48-hour fix.
What phosphates are
Phosphates are dissolved phosphorus compounds (mostly orthophosphate and polyphosphate) that occur naturally in tap water, fertilizer runoff, dead leaves, and many pool chemicals. They’re harmless to swimmers and don’t directly affect water clarity. But algae uses phosphate as a primary nutrient — given enough phosphate and even a tiny amount of sunlight, algae will outpace your chlorine’s ability to kill it.
Why your pool has them
- Tap water and fill water. Municipal water often contains 100–1,000 ppb phosphate from corrosion inhibitors added to protect city pipes.
- Fertilizer drift. Lawn fertilizer applied within 20 feet of the pool ends up in the water.
- Decomposing organic matter. Leaves, pollen, and dead bugs all break down into phosphate over weeks.
- Some pool chemicals. Stain & scale removers and metal sequestrants often contain phosphonates that break down to orthophosphate over time.
How much is too much?
The target for residential pools is under 100 ppb (parts per billion). Above 500 ppb, algae becomes very difficult to control even at high chlorine levels. Above 1,000 ppb, you’ll be in a losing fight with green algae regardless of how much shock you dose.
How to test for phosphates
Standard 5-way test kits don’t measure phosphate — you need a separate phosphate test kit, available as drops, strips, or photometer reagents. Most pool stores will test a water sample for free. If you’re fighting persistent algae, ask for a phosphate reading specifically.
How to remove phosphates
Phosphate remover (usually lanthanum chloride or aluminum sulfate based) binds dissolved phosphate into insoluble particles your filter can capture. The process:
- Test current phosphate level.
- Add phosphate remover per the label rate.
- Run the pump 24/7 for 24–48 hours.
- Clean or backwash the filter (captured phosphate solids will be visible as grey-white sludge).
- Retest. Below 100 ppb, you’re done.
One phosphate-remover treatment usually drops levels by 80–90%. For pools with chronic phosphate sources (well water, nearby fertilized lawn), the treatment may need to be repeated monthly.
Supporting chemicals for an algae-prone pool
Durachlor Assault 73 Shock
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Pool Season Water Clarifier
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Taylor K-2006 Test Kit
Shop NowShould every pool owner use phosphate remover routinely?
No. If your pool stays clear with normal chlorine and you’re not seeing recurring algae, you don’t need to add it. Phosphate remover is a tool for pools with chronic algae problems or known high-phosphate fill water — not a routine weekly chemical. Adding it when not needed wastes money and can drop calcium hardness slightly.
Send PST Pool Supplies a water test result if you’re fighting recurring algae, and we’ll match the right phosphate remover and shock combination for your pool size.