You’ve scrubbed the waterline, shocked the pool, and kept the chlorine steady. Yet somehow, algae keeps creeping back – faint green shadows along the walls, cloudy patches that won’t clear, or that stubborn film on the surface that reappears days after treatment.
The usual suspects – low chlorine, poor filtration, high pH – might not be the real culprit. Often, the problem lies with something invisible: phosphates.
Phosphates are nutrients that feed algae, turning your pool into an all-you-can-eat buffet for microscopic invaders. They don’t cause algae directly, but they accelerate growth, making it harder to maintain clear water – even when your sanitiser levels look fine.
For Perth pool owners, phosphates are a year-round concern. They enter pools through garden runoff, fertilisers, decaying leaves, and even some pool chemicals. Left unchecked, they create conditions where algae thrives, forcing pool owners into a cycle of constant treatment and frustration.
This guide explains what phosphates are, how they fuel algae, where they come from, and how to remove them effectively. It’s the kind of practical advice that helps pool owners understand the science behind water balance – and avoid the reactive, expensive fixes that come from chasing symptoms instead of solving the root cause.
What Are Phosphates and Why Do They Matter?
Phosphates are chemical compounds containing phosphorus, an essential nutrient for plant growth. In nature, phosphorus supports everything from crops to aquatic plants. In your pool, it does the same thing – except the “plants” are algae.
Algae need three things to grow: sunlight, water, and nutrients. Phosphates provide that third ingredient. When phosphate levels rise above 100 parts per billion (ppb), algae can multiply rapidly, even in pools with adequate chlorine.
The problem isn’t that phosphates kill chlorine or directly harm swimmers. They don’t. But they create an environment where algae can resist sanitiser more effectively, making it harder to keep water clear. Pool owners might find themselves adding more chlorine, running the pump longer, and still seeing algae return within days.
This is particularly common in Perth’s Western and Northern suburbs, where outdoor pools are surrounded by gardens, native bushland, and irrigation systems – all of which introduce phosphates through wind, rain, and runoff.
Where Do Phosphates Come From?
Phosphates enter pools from multiple sources, many of which are unavoidable in a typical backyard setup:
Garden Fertilisers and Soil Runoff: Lawn fertilisers, garden beds, and potting mix contain high levels of phosphorus. When it rains or irrigation systems overspray, phosphates wash into the pool. Even a light breeze can carry fertiliser dust onto the water surface.
Decaying Organic Matter: Leaves, pollen, grass clippings, and insects break down in the pool, releasing phosphates as they decompose. This is why pools near trees or native gardens often struggle with recurring algae, even with regular skimming.
Pool Chemicals and Treatments: Some algaecides, clarifiers, and scale removers contain phosphate-based compounds. These products work well for their intended purpose, but they can inadvertently raise phosphate levels, creating a feedback loop where the cure contributes to the problem.
Bore Water and Fill Water: Perth’s groundwater can contain naturally occurring phosphates, depending on the source. If pool owners top up with bore water or even some reticulated supplies, they might be adding phosphates without realising it.
Swimmers and Pool Use: Sunscreen, body oils, cosmetics, and sweat all contain phosphorus compounds. Heavy pool use – especially during summer – can gradually increase phosphate levels, even in well-maintained pools.
How Phosphates Fuel Algae Growth
Algae are opportunistic. They’ll grow wherever conditions allow, but phosphates give them a significant advantage.
When phosphate levels rise, algae can multiply faster than chlorine can kill them. This doesn’t mean chlorine isn’t working – it’s just overwhelmed. Think of it like trying to bail water from a sinking boat while someone keeps pouring more in. The bail bucket works fine; the problem is the rate of incoming water.
High phosphate levels also allow algae to form protective biofilms, making them harder to kill with standard sanitisers. This is why some pools develop persistent green patches or cloudy water that resists shock treatment. The algae aren’t invincible – they’re just well-fed.
Removing phosphates doesn’t replace chlorine or proper filtration. But it removes the fuel source, making it far easier for sanitiser to keep algae under control. It shifts the balance back in pool owners’ favour.
Testing for Phosphates: What You Need to Know
Most pool water tests focus on chlorine, pH, alkalinity, and calcium hardness. Phosphates aren’t usually part of the standard kit, which is why they often go unnoticed until algae becomes a recurring problem.
To test phosphate levels, pool owners need a phosphate test kit or test strips designed specifically for this purpose. These are available from pool shops or can be included in a professional water analysis.
0-100 ppb: Low risk. Algae growth is minimal, and standard chlorine levels keep water clear.
100-500 ppb: Moderate risk. Algae can establish itself, especially if chlorine drops or filtration is inconsistent.
Above 500 ppb: High risk. Algae growth is rapid, and pool owners struggle to maintain clarity without addressing phosphate levels first.
In Perth’s climate – where pools are exposed to garden runoff, high UV, and heavy use – phosphate levels above 200 ppb are common. Testing every few months, or whenever algae reappears despite proper chlorine levels, helps pool owners stay ahead of the problem.
For pool owners unsure how to interpret results or wanting a comprehensive water analysis, professional assessment can test phosphate levels as part of standard service checks, particularly for pools with recurring clarity issues.
How to Remove Phosphates from Your Pool
Phosphate removal is straightforward, but it requires the right product and proper follow-through. Here’s the process:
Step 1: Test Phosphate Levels
Confirm that phosphates are the issue. Don’t treat blindly – test first, then act based on results.
Step 2: Choose a Phosphate Remover
Phosphate removers work by binding to phosphates and pulling them out of solution, allowing the filter to capture them. Most products are lanthanum-based or aluminium-based compounds. Follow the manufacturer’s dosage instructions based on pool size and phosphate level.
Step 3: Add the Product with the Pump Running
Pour the phosphate remover slowly around the pool perimeter while the filtration system is running. This ensures even distribution and prevents localised cloudiness.
Step 4: Expect Temporary Cloudiness
As the remover binds to phosphates, the water may turn cloudy for 24-48 hours. This is normal. The cloudiness indicates that phosphates are being captured and removed by the filter.
Step 5: Clean or Backwash the Filter
Once the water clears, clean or backwash the filter to remove the captured phosphates. Cartridge filters should be rinsed thoroughly; sand and DE filters should be backwashed according to manufacturer instructions.
Step 6: Retest After 48 Hours
Confirm that phosphate levels have dropped. If they remain high, a second treatment may be necessary, particularly in pools with very high initial levels.
Phosphate removal doesn’t replace regular pool maintenance. It’s a targeted treatment that works alongside proper chlorination, pH balance, and filtration. For pools with persistent algae issues, it’s often the missing piece that finally brings water clarity under control.
Pool owners who prefer professional phosphate treatment as part of comprehensive residential pool care can have water testing, chemical balancing, and targeted treatments handled by experts.
Preventing Phosphate Build-Up
Once phosphates are under control, the goal is to keep them low. Here’s how:
Minimise Organic Debris: Skim leaves, pollen, and insects daily. Trim overhanging branches and keep garden beds away from the pool edge where possible. The less organic matter that enters the pool, the fewer phosphates are introduced.
Be Mindful of Fertiliser Use: If lawn or garden fertilisation happens near the pool, choose low-phosphate or phosphate-free options. Water gardens by hand rather than using sprinklers that overspray onto the pool area.
Test Fill Water: Pool owners who regularly top up with bore water should test it for phosphates before adding it to the pool. If levels are high, consider using mains water or treating the bore water before it enters the pool.
Clean the Filter Regularly: A clean filter captures organic matter before it decomposes and releases phosphates. Backwash sand and DE filters every few weeks; rinse cartridge filters monthly or as needed. Quality pool equipment maintenance ensures optimal filtration performance.
Shower Before Swimming: Encourage swimmers to rinse off sunscreen, body oils, and cosmetics before entering the pool. This simple habit reduces phosphate introduction and keeps water cleaner overall.
Use Phosphate-Free Chemicals Where Possible: Check product labels and choose algaecides, clarifiers, and other treatments that don’t contain phosphate compounds. Not all pool chemicals are phosphate-free, but many manufacturers now offer alternatives.
For property managers or strata committees overseeing community pools, phosphate control is particularly important. High bather loads and surrounding landscaping make these pools more vulnerable to phosphate build-up. Strata pool maintenance services include regular phosphate testing and treatment as part of compliance-focused care.
When to Call a Professional
Phosphate removal is manageable for most pool owners, but there are situations where professional help makes sense:
Phosphate levels above 1,000 ppb: High levels require multiple treatments and careful monitoring to avoid water balance issues.
Recurring algae despite treatment: If phosphates are low but algae keeps returning, there may be other underlying issues – equipment problems, poor circulation, or water chemistry imbalances.
Cloudy water that won’t clear: Persistent cloudiness after phosphate treatment could indicate filter problems, high calcium, or other factors that need professional diagnosis.
Rental properties or strata complexes: For landlords and property managers, outsourcing phosphate testing and treatment ensures consistent water quality without tenant complaints or compliance issues. Rental property pool services handle all testing and treatment, removing the burden from property managers.
Phosphate control is one part of a broader water management strategy. It’s most effective when combined with regular servicing, equipment checks, and proactive maintenance – not as a one-off fix.
The Role of Phosphates in Long-Term Pool Health
Phosphate management isn’t about achieving zero phosphates – that’s nearly impossible in an outdoor pool. It’s about keeping levels low enough that algae can’t gain a foothold, even during periods of heavy use or seasonal challenges.
In Perth’s climate, where pools are exposed to wind, pollen, and irrigation runoff year-round, phosphate control is a practical necessity. It reduces the need for excessive chlorine, cuts down on reactive shock treatments, and makes regular maintenance far more effective.
For homeowners who take pride in clear, balanced water, understanding phosphates is part of becoming a more informed pool owner. It’s the difference between constantly fighting algae and maintaining a pool that stays clear with minimal effort.
For those who prefer to leave the science to professionals, Indigo Pool Care offers comprehensive water testing and treatment as part of regular service visits. Professional technicians test for phosphates, adjust chemistry, and handle equipment checks so pool owners can focus on using the pool, not maintaining it.
Conclusion
Phosphates are the hidden variable in many algae problems. They don’t replace proper chlorination or filtration, but they make those systems far more effective by removing the nutrient source that algae rely on.
Testing for phosphates, treating high levels with a quality remover, and taking steps to prevent future build-up are straightforward actions that deliver long-term results. They’re particularly valuable in Perth’s outdoor pools, where garden runoff, native vegetation, and heavy summer use create ideal conditions for phosphate accumulation.
If pools struggle with recurring algae despite balanced chemistry, phosphates are worth investigating. A single test can reveal whether this invisible nutrient is the reason a pool won’t stay clear – and a single treatment can often resolve months of frustration.
For professional phosphate testing and treatment, or to arrange a comprehensive water analysis and service plan, contact us. Professional assessment can evaluate pool-specific conditions, recommend targeted solutions, and help maintain clear, healthy water without the guesswork.
