Nitrates & Phosphates in Pools - You wont believe this!
If your pool water looks clean but algae keeps returning no matter how much chlorine you add, the culprit is likely hiding in plain sight. Nitrates and phosphates are invisible nutrients that feed algae growth—and Perth pools are particularly vulnerable to both.
Understanding the difference between these two contaminants, where they come from, and how to treat them can save you endless frustration and chemical costs.
What Are Nitrates and Phosphates?
Both nitrates and phosphates are naturally occurring nutrients that act as fertilisers for algae. While they don't directly harm swimmers or damage pool equipment, they create the perfect conditions for algae to thrive—even when your chlorine levels appear adequate.
Phosphates are compounds containing phosphorus, typically entering pools through organic debris, certain cleaning products, and source water. They're measured in parts per billion (ppb), and levels above 500 ppb can promote algae growth.
Nitrates are nitrogen-based compounds that are significantly harder to remove than phosphates. They're measured in parts per million (ppm), and even low levels can fuel persistent algae problems. Nitrates are particularly concerning because standard pool chemicals cannot remove them.
The Key Differences
| Phosphates | Nitrates | |
|---|---|---|
| Measurement | Parts per billion (ppb) | Parts per million (ppm) |
| Problem threshold | Above 500 ppb | Above 10-20 ppm |
| Treatment difficulty | Moderate—removable with phosphate removers | Difficult—requires dilution or specialist products |
| Primary sources | Organic debris, fertilisers, cleaning products, bore water | Lawn fertiliser runoff, bird droppings, contaminated groundwater |
Sources and Causes
Where Phosphates Come From
Phosphates accumulate in Perth pools through multiple pathways:
Organic debris: Leaves, pollen, grass clippings, and soil all contain phosphorus. As this material breaks down in your pool, it releases phosphates into the water. Perth's paperbark trees and eucalypts are particularly heavy contributors during spring and autumn.
Bore water and scheme water: Perth's water supply can contain measurable phosphate levels. If you're topping up regularly during summer or using bore water, you're continuously adding phosphates.
Garden fertilisers: Phosphorus is a key ingredient in lawn and garden fertilisers. Overspray, irrigation runoff, and wind can carry fertiliser particles directly into your pool.
Cleaning products: Some household cleaners and pool tile cleaners contain phosphates. Even small amounts introduced during cleaning can accumulate over time.
Swimmers: Skin cells, sunscreen, body oils, and cosmetics all contribute phosphates to pool water.
Where Nitrates Come From
Nitrates are often the overlooked problem in Perth pools, and lawn fertiliser runoff is the primary culprit:
Lawn fertiliser runoff: This is the biggest source of nitrates in residential pools. Nitrogen-rich fertilisers applied to lawns and gardens wash into pools during rain or irrigation. Perth's sandy soils don't absorb and hold nutrients well, meaning fertiliser moves easily across the surface and into pool water. Even a narrow strip of lawn between your pool fence and garden can channel significant nitrates into the pool.
Bird and animal droppings: Bird waste is extremely high in nitrogen. A few magpies or corellas visiting your pool area regularly can introduce substantial nitrate loads over time.
Contaminated groundwater: In areas with agricultural history or heavy fertiliser use, bore water can contain elevated nitrate levels. Parts of Perth's northern suburbs and semi-rural areas are particularly affected.
Decomposing organic matter: As leaves, insects, and other debris break down, the nitrogen they contain converts to nitrates. Pools surrounded by trees or with poor skimming habits accumulate nitrates steadily.
Chlorine stabiliser breakdown: Cyanuric acid (stabiliser) contains nitrogen. While it doesn't directly become nitrate under normal conditions, heavily over-stabilised pools with other nitrogen sources can see elevated levels.
How to Test for Nitrates and Phosphates
Standard pool test kits don't measure nitrates or phosphates, which is why these problems often go undiagnosed for months.
If you're experiencing persistent algae despite maintaining proper chlorine and pH levels, bring a water sample to Poolwise for comprehensive testing. We can measure both phosphate and nitrate levels and identify which nutrient is driving your algae problem.
Signs that suggest nutrient contamination include:
- Algae returns within days of treatment
- Chlorine demand seems unusually high
- Water has a slight green tinge that won't clear
- Pool is near heavily fertilised lawns or gardens
- Birds frequently visit the pool area
Treatment Options
Treating Phosphates
Phosphate removal is relatively straightforward with the right products. We recommend Lo-Chlor's phosphate removal range for Perth pools:
Lo-Chlor Starver Pool Algae Starver: Our go-to phosphate remover for most situations. It binds to phosphates and drops them to the pool floor where they can be vacuumed to waste. One litre treats up to 50,000 litres of pool water with phosphate levels around 1000 ppb.
Lo-Chlor Starver X: A concentrated formula for pools with very high phosphate levels or persistent problems. Ideal when phosphates exceed 2000 ppb or when you need rapid reduction before an event or holiday.
Lo-Chlor Starver Xtreme: The most powerful option for severe phosphate contamination. We recommend this for pools that have been neglected or where multiple high-phosphate sources are present.
Application tips:
- Apply with the filter running
- Allow 24-48 hours for the product to work
- Vacuum the white precipitate to waste (not through the filter)
- Retest after treatment to confirm reduction
Treating Nitrates
Nitrates are trickier because no standard pool chemical breaks them down. Your options are:
Dilution: The traditional approach is to partially drain and refill the pool with fresh water. This works but wastes water and chemicals. You may need to drain 50% or more to significantly reduce nitrate levels.
Lo-Chlor Miraclear Clarifier Cubes: While not a nitrate remover per se, this product helps manage the consequences of nitrate contamination by keeping water crystal clear and reducing the organic load that feeds algae. Use it as part of an ongoing maintenance program for pools prone to nitrate issues.
Prevention focus: Because nitrate removal is difficult, prevention becomes essential. Address the sources (see below) rather than trying to treat the symptom repeatedly.
Prevention Strategies
Reducing Nitrate Entry
Manage lawn fertiliser carefully: Apply fertiliser to lawns in the late afternoon when irrigation has finished for the day. Never fertilise before rain. Consider slow-release fertilisers that are less prone to runoff. Keep fertiliser application at least 2 metres from pool fencing.
Create a buffer zone: If possible, establish a strip of unfertilised ground, paving, or gravel between lawn areas and your pool fence. This catches runoff before it reaches the pool.
Address drainage: Ensure garden beds and lawns drain away from the pool, not toward it. A simple regrade or the addition of drainage channels can make a significant difference.
Deter birds: While you can't eliminate birds entirely, removing overhanging branches and installing visual deterrents can reduce the frequency of visits and associated droppings.
Use a pool cover: A quality pool cover dramatically reduces debris entry, bird access, and contamination from all sources.
Reducing Phosphate Entry
Skim and clean regularly: Remove leaves and debris before they break down and release phosphates.
Rinse before swimming: A quick rinse removes sunscreen, cosmetics, and body oils that contribute phosphates.
Check your water source: If using bore water, have it tested for phosphate content. You may need to treat phosphates as part of routine maintenance rather than just when problems arise.
Maintain your filter: A clean, efficient filter removes organic particles before they decompose. Backwash or clean cartridges according to manufacturer recommendations.
Ongoing Maintenance
For pools with chronic nutrient problems, we recommend a preventive maintenance approach:
Monthly phosphate treatment: A maintenance dose of Lo-Chlor Starver every 4-6 weeks keeps phosphate levels from accumulating to problem levels. This is far easier than dealing with algae blooms after the fact.
Regular testing: Include phosphate testing in your routine water analysis, particularly during spring when organic debris peaks and after heavy fertiliser application to surrounding gardens.
Source management: Work on eliminating or reducing contamination sources. This is more effective long-term than repeatedly treating symptoms.
Need Help?
Some pool shops don't test for phosphates, and it is not at all common to test for nitrates. Persistent algae problems are frustrating, but they're usually solvable once you identify whether nitrates, phosphates, or both are responsible. Bring a water sample to Poolwise for comprehensive testing, and we'll help you develop a treatment and prevention plan tailored to your pool's specific situation.
We stock the full range of Lo-Chlor phosphate removers and can advise on the most cost-effective approach for your contamination level. For pools with nitrate issues, we can help you understand your options and develop strategies to reduce ongoing contamination from lawn runoff and other sources.
