Phosphates, Nitrates and Pool Algae: A Research Review

Published November 13, 2026 by A Clean Pool USA

This is the long version of our post Why a Balanced Pool Still Grows Algae. It collects the evidence behind each point, says where each piece of evidence comes from, and is plain about where the evidence runs out. Numbers in square brackets point to the references at the end. Every source was checked on November 13, 2026. For some journal articles we read the published abstract rather than the full text.

Summary

• Free chlorine on a test kit includes chlorine held by stabilizer (cyanuric acid). In stabilized water, more than 99 percent of available chlorine is in that held form [1]. Disinfection tracks the active form, hypochlorous acid, which is set by the ratio of stabilizer to chlorine, not by the chlorine reading alone [3].
• The CDC's 2024 Model Aquatic Health Code caps stabilizer at 90 ppm in public pools and lists a stabilizer to chlorine ratio above 45:1 as an imminent health hazard [4]. A 2019 peer-reviewed risk model recommends a maximum ratio of 20:1 [3]. Florida's public pool rule caps stabilizer at 100 mg/L, with 40 mg/L as the recommended maximum [5].
• Stabilizer leaves a pool only with water. Oxidizing it with chlorine would take "prohibitively high" chlorine levels [2]. Draining and refilling is the method the CDC code itself names [4].
• Whole-lake experiments show phosphorus controls algae growth in fresh water [6], and later work argues both nitrogen and phosphorus need control [7]. Algae can store surplus phosphorus for later use [8]. These findings come from lakes and labs, not pools.
• Lanthanum phosphate is among the least soluble phosphate salts known [9], and lanthanum compounds do remove phosphate from pool water [10] and lakes [11]. Dissolved carbonate competes for the lanthanum [10].
• Nitrate cannot be oxidized further by chlorine and has no practical chemical removal in a pool. Reverse osmosis, ion exchange or dilution are the options [18].
• We found no peer-reviewed study that tests phosphate or nitrate levels against algae growth in chlorinated pools. That gap is a large part of why the debate continues.

How to Read This Review

Each claim is labeled with the kind of source behind it, because a whole-lake experiment, a building code and a product blog do not carry the same weight:

• Peer-reviewed: published in a scientific journal after review by other scientists.
• Industry journal: the Journal of the Swimming Pool and Spa Industry, written by working chemists for the pool trade.
• Code or regulation: the CDC's Model Aquatic Health Code, the Florida Administrative Code, or a city ordinance.
• Utility report: a water utility's own annual water quality report.
• Patent: a granted U.S. patent. Patents report experiments, but they are written to support a claim, not reviewed like a paper.
• Vendor: a pool chemical company's published guidance. Useful for practice, but the company sells the product.
• Our calculation and our practice: arithmetic we did ourselves (shown in full), and how A Clean Pool USA works on its routes.

1. Chlorine, Stabilizer and the Active Fraction

Cyanuric acid is added to outdoor pools because it slows the loss of chlorine to sunlight. Wojtowicz reports a sunlight loss rate of "only about 1% per day at 138 ppm" stabilizer, and explains why: "over 99% of the av. Cl in CA stabilized pools is in the form of chloroisocyanurates," compounds that do not absorb much ultraviolet light [1] (industry journal).

The protection costs speed. Using laboratory data, the same paper calculates the minimum free chlorine needed to kill 99 percent of a test bacterium (Streptococcus faecalis) in 30 seconds at pH 7.5 and 85°F, and shows that the required chlorine rises as stabilizer rises [1] (industry journal). That calculation is for bacteria, not algae, but the mechanism is the same: the chlorine held by stabilizer is a reserve, and the active form, hypochlorous acid (HOCl), does the work.

Falk and colleagues reanalyzed published disinfection data and found that inactivation of Cryptosporidium correlated far better with HOCl concentration than with free chlorine as measured (r = −0.96 versus −0.06). They also found that HOCl varies only about ±1.8 percent when the stabilizer to chlorine ratio is held constant over typical pool ranges, and that the free chlorine and stabilizer limits allowed in 27 U.S. states produce HOCl levels "spanning more than a factor of 500." They recommend regulating the two together as a ratio, with "a maximum CYA/FC ratio of 20" [3] (peer-reviewed). The study models illness risk from pathogens. It does not test algae.

What the codes say

These rules are written for public pools. Chapter 64E-9 covers public swimming pools, not private residential pools, and the model code is a template that states and counties may adopt. They are still the best published benchmarks for a home pool.

A worked example (our calculation): a pool at 100 ppm stabilizer, the Florida public pool maximum, with 3 ppm free chlorine sits at a ratio of about 33:1. That passes the 45:1 hazard line and fails the 20:1 recommendation. Some pool care guidance uses an even tighter rule of thumb aimed at algae, free chlorine at 7.5 percent of stabilizer, a ratio of about 13:1 (one example is [19], vendor). By that rule, the same pool would need about 7.5 ppm chlorine at all times.

2. Stabilizer Only Leaves With Water

Where it comes from

Trichlor tablets (trichloroisocyanuric acid) and dichlor (sodium dichloroisocyanurate) release stabilizer as they release chlorine. The arithmetic (our calculation):

• Trichlor, C₃N₃O₃Cl₃, molar mass about 232.4 g. Each molecule releases three HOCl, worth 3 × 70.9 = 212.7 g of available chlorine (expressed as Cl₂), and leaves one cyanuric acid, 129.1 g.
• Stabilizer added per unit of chlorine: 129.1 ÷ 212.7 = about 0.61. Every 1 ppm of chlorine from trichlor adds about 0.6 ppm of stabilizer.
• The same arithmetic gives about 91.5 percent available chlorine for trichlor, which matches its usual label value of about 90 percent, and shows that cyanuric acid is about 55 percent of a tablet's weight (129.1 ÷ 232.4).
• Dichlor (anhydrous, about 220.0 g) releases two HOCl (141.8 g of available chlorine) per cyanuric acid: 129.1 ÷ 141.8 = about 0.9 ppm stabilizer per 1 ppm chlorine.

Liquid chlorine (sodium hypochlorite), calcium hypochlorite and salt chlorine generators add no stabilizer.

Why it does not go away

Wojtowicz tested whether excess stabilizer could be reduced by treating the water with chlorine. The study "showed that prohibitively high concentrations of available chlorine would be required to achieve significant reductions in cyanuric acid levels in a practical time," and that the loss of stabilizer under typical pool conditions "is probably not significant" [2] (industry journal).

The CDC model code reflects the same fact. Its procedure for disinfecting a stabilized pool after a diarrheal contamination incident includes "lowering the CYA concentration to less than or equal to 15 ppm (mg/L) by draining and then refilling with water from the approved water source," if necessary (6.5.3.2.1) [4] (code). When the code needs stabilizer lowered, draining is the tool it names.

Wojtowicz also notes that "many local health codes limit CA to 100 ppm and some do not allow use of CA at all" [1].

3. Algae and Nutrient Limitation

Algae need light, warmth, carbon dioxide and mineral nutrients. Of the nutrients, phosphorus and nitrogen are the two most often in short supply in fresh water, and whichever is scarcest limits growth.

The best-known evidence is Schindler's whole-lake experiments in northwestern Ontario. In Lake 226, two similar basins were separated at a shallow neck. Beginning in late May 1973, both basins received nitrogen and carbon, but only one received phosphorus. By September a bloom of the blue-green alga Anabaena spiroides covered the phosphorus basin, while phytoplankton in the basin that received only nitrogen and carbon "remained similar to those before fertilization was begun." The abstract adds that "recovery was almost immediate when phosphate additions only were discontinued" in another lake [6] (peer-reviewed).

Later work argued that phosphorus alone is not the whole story. Conley and colleagues concluded that "improvements in the water quality of many freshwater and most coastal marine ecosystems requires reductions in both nitrogen and phosphorus inputs" [7] (peer-reviewed). For a pool, that supports watching both nutrients, not only phosphate.

Algae also stockpile. Microalgae practice "luxury uptake," defined as "the uptake of P beyond that required for growth and storage of phosphate within the biomass as PolyP" (polyphosphate), and they hold those stores for days [8] (peer-reviewed review). So a brief drop in phosphate does not starve algae that have already stocked up. Keeping phosphate low and keeping it there matters more than one large treatment.

The limit of this evidence. Lakes and lab cultures are not chlorinated. In a pool, chlorine kills algae directly, and nutrients matter only during the hours when chlorine is too weak to keep up. We found no peer-reviewed study that measures how phosphate or nitrate levels change algae regrowth in a chlorinated, stabilized pool. The case for nutrient control in pools rests on the lake science above, the chemistry of chlorine and stabilizer, and field experience.

4. How Phosphate Removers Work

Many pool phosphate removers are lanthanum compounds; the product label lists the active ingredient. Lanthanum reacts with dissolved phosphate to form lanthanum phosphate, a solid. Firsching and Brune measured the solubility of the rare earth phosphates at 25°C and found "the least soluble ones are lanthanum phosphate (pK = 26.15)" and ytterbium phosphate [9] (peer-reviewed). A pK of 26 means the solid essentially does not redissolve.

A pool-specific patent describes "methods and compositions for removing phosphate from swimming pool waters," with the lanthanum phosphate and carbonate solids "substantially all retained in the filter." It also reports a practical catch: when soluble lanthanum chloride is used, "a significant proportion of the lanthanum chloride reacts with dissolved carbonate," which slows phosphate removal [10] (patent). Pool water carries carbonate as part of its alkalinity, so some of any dose goes to carbonate rather than phosphate. That is one reason a single small dose can disappoint, and why the filter needs cleaning afterward.

The same chemistry is used at lake scale. A lanthanum-modified clay has been applied in "around 200 environments worldwide," and a review found "a high efficiency for phosphorus binding," which "can be limited by the presence of humic substances and competing oxyanions" [11] (peer-reviewed review).

5. Phosphate and Nitrate Sources in Central Florida

Drinking water

Many utilities add phosphate compounds to drinking water to control corrosion of pipes. Five water systems in our service area name a phosphate product in their own documents (utility reports):

Toho's notice is the only one that prints a number. It reports that one Bellalago sample in August 2026 measured 0.14 mg/L, "below the required 0.7 mg/L for orthophosphate" [20]. The notice does not say whether the figure is expressed as phosphate or as phosphorus, so we do not convert it. Even so, a required minimum of 0.7 mg/L (700 parts per billion, as printed) is above the 500 ppb phosphate guideline that pool care sources often cite [19], before any rain, fertilizer or debris. The other four reports name the product but not the dose.

Other utilities may also use phosphate. Our October 28, 2026 pool water by utility guide recorded hardness, pH and alkalinity, not corrosion control, so a utility missing from this table has not been checked for it.

Fertilizer

The City of Maitland "prohibits the use of lawn fertilizers containing nitrogen and/or phosphorus between June 1 and September 30" to protect its lakes [17] (city ordinance). The rule exists because lawn fertilizer reaches water during the rainy season. A pool next to a fertilized lawn is in the path of the same runoff and overspray.

Pool products

Some water treatment products, including some stain and scale preventers, contain phosphonates such as HEDP; the label or safety data sheet will say. A Nalco patent describes "reputedly chlorine-resistant phosphonates such as 1,1-hydroxyethylidine diphosphonic acid (HEDP)" breaking down under chlorination in recirculating cooling water, and notes that "the orthophosphate which is a product of this reaction can contribute to scaling problems" [12] (patent). The patent studied cooling towers under heavy chlorination, not pools, so it shows the reaction is real without showing how much phosphate a pool product adds. A pool chemical maker also lists "fertilizers, cleaning products, and even swimming pool algaecides" as common sources of nitrogen compounds [18] (vendor).

Debris and swimmers

Leaves, pollen, grass clippings and soil carry both nutrients. Swimmers add nitrogen compounds such as urea [18] (vendor).

6. Nitrate

Chlorine oxidizes nitrite to nitrate, but nitrate is nitrogen's most oxidized form and cannot be oxidized further. In the words of one pool chemical company, "chlorine oxidizes nitrite into nitrate, but we're still left with nitrates, which cannot be further oxidized," and "nitrates cannot be removed by filtration other than reverse osmosis (RO) and ion exchange." When those are too costly, "draining and diluting your pool may be the best alternative" [18] (vendor). The chemistry here is standard; the vendor source is cited for the pool-specific framing.

We did not find a published, peer-reviewed nitrate threshold for algae in pools, and we do not quote one.

7. Where the Skeptics Are Right, and Where Practice Differs

The argument that phosphate removers are a waste of money goes like this: if free chlorine is kept high enough for the stabilizer level at all times, algae die faster than they can grow, and nutrient levels do not matter. The evidence above supports that chemistry. Even a phosphate remover maker agrees. Its guidance quotes Richard Falk that "phosphate removers are a good preventative measure, though not required," says that at a free chlorine to stabilizer ratio of 0.075:1 "chlorine should be able to stay ahead of algae," and says phosphate removal "does not undo an outbreak of algae" [19] (vendor).

The same page then makes the point this review turns on: holding that ratio "can be very difficult and impractical to do, especially if your pool is overstabilized" [19]. The disagreement is about the words "at all times." On a Florida route with weekly visits, chlorine demand swings with heat, sun, rain, swimmers and debris, and tablet feeders keep raising stabilizer, which raises the chlorine needed. Lowering stabilizer narrows the chlorine target. Lowering nutrients slows algae during the hours chlorine falls short. Both widen the margin. Neither replaces chlorine, and neither clears a pool that is already green.

The same reasoning covers drain and refill. For stabilizer and nitrate, there is no chemical alternative: replacing water is the documented method [2][4][18]. It is not a cure for every cloudy pool, but when stabilizer or nitrate is the problem, it is the fix, not a waste.

8. Putting It Together

What the evidence above points to, for a pool that keeps growing algae while chlorine reads "good":

• Test stabilizer, phosphate and nitrate in addition to chlorine, pH and alkalinity.
• Lower stabilizer by partial drain and refill when it is high, and hold chlorine in proportion to stabilizer.
• Treat phosphate with a lanthanum-based remover, then clean the filter.
• Dilute nitrate by partial drain and refill when it is high.
• Do not fully drain a pool without checking groundwater first. Central Florida's water table is high, and groundwater pressure can lift an empty or nearly empty shell.

Our practice: A Clean Pool USA does partial drains and refills when stabilizer or nitrate calls for it, and checks groundwater conditions before taking the water level down.

9. What We Could Not Find

• A peer-reviewed study testing phosphate or nitrate levels against algae growth in chlorinated, stabilized pools.
• A peer-reviewed phosphate or nitrate threshold for pools. The commonly quoted 500 ppb phosphate guideline appears in vendor guidance, which attributes "phosphates over 500 ppb can be problematic" to Richard Falk [19]. We found no study behind the number.
• Measured phosphate concentrations in local tap water. Only Toho's Bellalago notice prints a number, and it is a regulatory minimum, not a typical reading.

If you know of research we missed, tell us and we will add it.

References

1. Wojtowicz, J.A. "Effect of Cyanuric Acid on Swimming Pool Maintenance." Journal of the Swimming Pool and Spa Industry 5, no. 1 (2004): 15 to 19. PDF. Industry journal.
2. Wojtowicz, J.A. "Oxidation of Cyanuric Acid with Hypochlorite." Journal of the Swimming Pool and Spa Industry 4, no. 2 (2001): 23 to 28. PDF. Industry journal.
3. Falk, R.A., Blatchley, E.R., Kuechler, T.C., Meyer, E.M., Pickens, S.R., and Suppes, L.M. "Assessing the Impact of Cyanuric Acid on Bather's Risk of Gastrointestinal Illness at Swimming Pools." Water 11, no. 6 (2019): 1314. doi:10.3390/w11061314. Peer-reviewed.
4. Centers for Disease Control and Prevention. 2024 Model Aquatic Health Code, Code Language, 5th edition, December 2024. Sections 5.7.3.1.1.2.2, 5.7.3.1.3.2, 5.7.3.1.3.3, 6.5.3.2.1 and 6.6.3.1. CDC MAHC page. Code.
5. Florida Administrative Code, Rule 64E-9.004(1)(d)4, Operational Requirements (public swimming pools), as amended effective June 11, 2023. Text via Cornell LII. Regulation.
6. Schindler, D.W. "Eutrophication and Recovery in Experimental Lakes: Implications for Lake Management." Science 184, no. 4139 (1974): 897 to 899. doi:10.1126/science.184.4139.897. Peer-reviewed.
7. Conley, D.J., Paerl, H.W., Howarth, R.W., Boesch, D.F., Seitzinger, S.P., Havens, K.E., Lancelot, C., and Likens, G.E. "Controlling Eutrophication: Nitrogen and Phosphorus." Science 323, no. 5917 (2009): 1014 to 1015. doi:10.1126/science.1167755. Peer-reviewed.
8. Slocombe, S.P., Zúñiga-Burgos, T., Chu, L., Wood, N.J., Camargo-Valero, M.A., and Baker, A. "Fixing the Broken Phosphorus Cycle: Wastewater Remediation by Microalgal Polyphosphates." Frontiers in Plant Science 11 (2020): 982. doi:10.3389/fpls.2020.00982. Peer-reviewed review.
9. Firsching, F.H., and Brune, S.N. "Solubility Products of the Trivalent Rare-Earth Phosphates." Journal of Chemical and Engineering Data 36, no. 1 (1991): 93 to 95. doi:10.1021/je00001a028. Peer-reviewed.
10. Mills, D.J. (Innovez Pty Ltd). "Removal of Phosphate from Water." U.S. Patent 6,846,432 B2, January 25, 2005. Google Patents. Patent.
11. Copetti, D., Finsterle, K., Marziali, L., Stefani, F., Tartari, G., Douglas, G., Reitzel, K., Spears, B.M., Winfield, I.J., Crosa, G., D'Haese, P., Yasseri, S., and Lürling, M. "Eutrophication Management in Surface Waters Using Lanthanum Modified Bentonite: A Review." Water Research 97 (2016): 162 to 174. doi:10.1016/j.watres.2015.11.056. Peer-reviewed review.
12. Johnson, D.A. (Nalco Chemical Co.). "Method for Prevention of Phosphonate Decomposition by Chlorine." U.S. Patent 4,642,194, February 10, 1987. Google Patents. Patent.
13. City of Sanford. 2026 Annual Drinking Water Quality Report (January to December 2025 data). Utility report.
14. City of Altamonte Springs. 2025 Annual Drinking Water Quality Report. Utility report.
15. City of Casselberry. 2025 Consumer Confidence Report. Utility report.
16. City of Deltona. 2025 Water Quality Report. Utility report.
17. City of Maitland. Lakes Management (fertilizer restrictions), checked November 13, 2026. City ordinance summary.
18. Orenda Technologies. Nitrates, Nitrites, and Urea in Swimming Pools. Vendor.
19. Orenda Technologies. Phosphate Removal (Pillar 3). Vendor.
20. Toho Water Authority. Bellalago Orthophosphate Level Public Notice, September 5, 2026 (PDF). Utility notice.

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