What Are Phosphates in a Pool — and Why Does It Matter?

If you've ever had a pool that kept turning green even though your chlorine level looked fine, phosphates are probably the culprit. Most pool owners have never heard of them — and most pool companies never test for them. Here's everything you need to know.

What Are Phosphates?

Phosphates are naturally occurring compounds (phosphate ions) that dissolve in water. On their own, they're harmless to swimmers — they don't irritate skin, they don't affect pH, and they don't smell. The problem is what they do to algae.

Algae needs three things to grow: sunlight, CO2, and nutrients. Phosphates are algae's favorite food. A pool with high phosphates is essentially a buffet for algae — and no matter how much chlorine you add, you're fighting an uphill battle if the food source is still there.

Where Do Phosphates Come From?

This is the part that surprises most people — phosphates enter your pool from everywhere:

• Leaves and plant debris — the biggest source for most Inland Empire pools. Decomposing organic material releases phosphates directly into the water.
• Tap water — Riverside and surrounding cities add phosphate-based corrosion inhibitors to municipal water. Every time you fill or top off your pool, you're adding phosphates.
• Fertilizer runoff — if you have a lawn near your pool, wind and rain carry fertilizer (which is loaded with phosphates) into the water.
• Sunscreen and body products — swimmers bring in trace amounts with every swim.
• Pool chemicals themselves — some cheaper chlorine tablets and shock products contain phosphate-based fillers.
• Rain — especially in areas like the Inland Empire where desert dust carries organic material.

In short, if your pool is outside (and hopefully it is), phosphates are constantly entering the water. The question is whether you're managing them.

What's a Normal Phosphate Level?

Phosphates are measured in parts per billion (ppb). Here's how to read your results:

Phosphate Level	What It Means
0 – 100 ppb	Ideal — algae has almost nothing to feed on
100 – 500 ppb	Elevated — algae risk increases, treat soon
500 – 1,000 ppb	High — chlorine efficiency is noticeably reduced
1,000+ ppb	Very high — chronic algae problems likely regardless of chlorine level

Many pools we service in Riverside, Corona, and Norco come in well above 1,000 ppb — especially if they've never been tested for phosphates before. It's one of the most overlooked readings in residential pool care.

How Do You Test for Phosphates?

Standard test strips don't test for phosphates. Most basic pool test kits don't either. You need a dedicated phosphate test kit or test strips (LaMotte and Taylor both make good ones), or you can bring a water sample to a pool supply store that offers free water testing — most do.

At No Excuses Pool Service, we test for phosphates as part of our 21-point pool inspection because it's one of the leading causes of chronic algae problems that homeowners can't figure out on their own.

How Do You Remove Phosphates From a Pool?

The good news: there's a straightforward fix. Phosphate removers bind to phosphate molecules and cause them to clump together so your filter can trap and remove them.

The most widely used product is Natural Chemistry PHOSfree — it's effective, safe, and works with all pool types. Here's how the treatment works:

1. Test your phosphate level to know how much product to use (dosage is based on pool volume and phosphate concentration)
2. Shake the bottle well — PHOSfree is a lanthanum-based compound that settles
3. Add it slowly near a return jet with the pump running
4. Run your filter continuously for 48 hours — this is critical, the filter is doing the actual removal
5. Clean or backwash your filter after treatment, as it will trap the phosphate compounds
6. Retest — very high levels may need a second treatment

Do I Need to Treat Phosphates Every Month?

Not necessarily. For most pools with good maintenance, a phosphate treatment 2–4 times per year is enough to keep levels in check. Pools surrounded by heavy landscaping, pools that get a lot of debris, or pools in areas with heavy fertilizer use may need more frequent treatment.

The key is testing regularly — at least once a month during swim season — so you catch levels before they get out of control. A phosphate level that's managed at 200 ppb is a 10-minute fix. A pool sitting at 2,000+ ppb after two years of neglect is a whole different job.

Phosphates vs. Chlorine — The Connection

Here's the practical impact: at high phosphate levels, algae becomes resistant to normal chlorine doses. You can have a free chlorine reading of 3–5 ppm (which is correct) and still get an algae bloom if phosphates are sky high. This is why so many pool owners end up in a cycle of shocking their pool repeatedly with no lasting results.

Fixing the phosphate level doesn't replace your need for chlorine — you still need both. But once phosphates are under control, your chlorine does its job properly and you stop fighting losing battles with algae.

The Bottom Line

Phosphates are a silent problem — they don't make your pool look bad on their own, but they set the stage for chronic algae issues that are frustrating and expensive to deal with. Testing for them, treating when needed, and keeping your filter clean are the three steps that make the biggest difference.

Most pool owners never know phosphates are an issue until they've already been battling algae for months. Now you do. If you'd like us to test your pool's phosphate levels during a service visit across Riverside, Corona, Norco, Eastvale, or Jurupa Valley, we include it in every 21-point inspection.