Phosphates in Your Pool in Los Angeles: When They Matter

Phosphate is phosphorus bound to oxygen, the same nutrient sold in a bag of lawn fertilizer. In a pool it does nothing to swimmers, plaster or equipment. Its one effect is on algae: once chlorine stops holding algae back, phosphate lets a bloom grow faster and come back sooner.

That is the part the remover ads leave out. A pool with chlorine where it belongs stays clear at 1,000 ppb (parts per billion), and a pool with none goes green at 100. The reading tells you how fast things go wrong when the sanitizer fails. It does not tell you that they will.

Do you need to remove phosphates from your pool?

Not every time. Read the number together with what the water has been doing this season.

• Under 500 ppb. Leave it alone. Keep chlorine in range and spend nothing on remover.
• 500 to 1,000 ppb. Worth treating if algae has come back more than once this year, or if the pool goes a week at a time without anyone testing it.
• Over 1,000 ppb. Treat it, and do it before summer. At this level one hot weekend of weak chlorine in July is enough for a bloom.
• Several thousand ppb. Something feeds the pool every day: a sprinkler, a planter that drains into it, ash or fire retardant. Find that first, or you will buy the remover twice.

Where pool phosphates come from in Los Angeles

It gets in a little at a time, and most of it rides in on something you can see.

• Lawn and sprinklers. Valley yards in Encino and Sherman Oaks run fertilized grass right up to the coping, and a sprinkler that throws part of its fan into the pool rinses a little fertilizer in on every cycle. Grass clippings washed off the lawn add more.
• Leaves. Sycamore, oak and jacaranda drop into foothill pools most of the year. A leaf that sinks and rots on the floor gives its phosphate to the water, and so does a skimmer basket left full for a week.
• Dust and ash. Santa Ana winds carry fine organic dust across every pool in their path. Ash and pink fire retardant carry far more, and the wildfire ash guide covers that case on its own.
• Tap water. Some water systems add phosphate to keep pipes from corroding. Your supplier's yearly water quality report says whether yours does, and summer top-offs bring it in an inch at a time.
• Pool products. Several stain and scale preventers (sold as metal sequestrants) are phosphonate-based and break down into phosphate over the weeks after a dose. Sunscreen and lotion add a little too.

How to test for phosphates

Standard strips and the usual chlorine-and-pH kit skip phosphate. It needs its own test, a strip or a drop kit that turns the sample blue, darker for more. Most pool supply stores will also run it free on a bottle of your water.

Two things throw the reading off. Phosphate tests read wrong when chlorine is far above normal, so test before a shock and not the morning after. And a test taken right after a dose of remover shows phosphate the filter has not caught yet. Give it a week.

How to remove phosphates, in order

Most removers on the shelf are lanthanum-based. Lanthanum locks phosphate into particles so fine they cloud the water, and then the filter has to catch them. So the filter decides how well the remover works, and the remover goes in last, not first.

1. Clear any algae first. Remover takes away the food. It does not kill what is already growing, so green water goes through the shock routine in the green pool clean-up guide before anything else.
2. Clean the filter. A dirty filter cannot hold the fine particles and sends them straight back into the pool.
3. Stop the source. Turn the sprinkler, empty both baskets, net the leaves off the floor.
4. Dose by the label, for your reading and your gallons. A 15 × 30 foot pool with an average depth of 5 feet holds about 17,000 gallons. Above a few thousand ppb, split the dose over two or three days.
5. Run the pump around the clock for a day or two. The water may go hazy for a while. That haze is the phosphate coming out.
6. Clean the filter again. Hose the cartridge, or backwash a sand or DE filter. The grey paste in the pleats is what you paid to remove.
7. Retest after a week. If the number barely moved, the source is still running.

Too much phosphate remover: what happens

Mostly cloudy water and a filter that clogs fast. Lanthanum that finds nothing left to bind floats as a milky haze until the filter strains it out, and a heavy dose can load a cartridge within hours. If the pool turns milky after treatment, keep the pump running, clean the filter whenever the pressure gauge climbs 8–10 psi above its clean reading, and add nothing more. Swim again once the water is clear.

What phosphate treatment costs

Remover is a store product, so these numbers are retail prices from stores and online shops in November 2026, not ours: about $19–$40 for a quart, around $110 for a gallon of concentrate. What moves the total is the reading, the size of the pool and whether it needs a second round because the source kept running.

On our weekly service, phosphate is on the chemical panel at every visit, so it gets caught while it climbs rather than after the pool turns. Weekly service starts at $159 a month, and remover is billed separately on the visits that need it, as the price list shows.

Keeping phosphates low

• Aim sprinklers away from the water, and sweep fertilizer granules off the deck before they get hosed in.
• Empty the skimmer and pump baskets every week, twice a week in leaf season.
• Vacuum the floor so sunken leaves come out before they rot.
• Hold chlorine in range. That is what stops algae. Low phosphate only buys you time when the chlorine slips.

Not sure what your number means? Send the test result and a photo of the water, or call (818) 740-6099.