Green pool algae is the most common reason pool owners across Perth call us. The pool water turns green, the pool walls and pool floor feel like slippery surfaces, and the swimming pool becomes unsafe. After two decades of green pool recovery from Duncraig to Ocean Reef, Adrian can spot what went wrong before the test results come back. This step by step guide covers what causes pool algae, how to fix a green pool, and the pool algae prevention habits that keep your pool algae free year round.
What Is Pool Green Algae
Green algae is the most common pool algae and the main cause of green pool water. It spreads when chlorine levels drop, ph levels drift, or poor water circulation leaves dead spots where algae thrives. Algae spores arrive on wind, rain and swimmers, and in hot weather they can bloom within a few hours. During photosynthesis algae consumes carbon dioxide from the water, which raises ph and further reduces chlorine effectiveness. That cycle is why a small green tint can become a full algae infestation over a single weekend in Perth.
Why Does A Pool Turn Green
Pools turn green when free chlorine cannot keep up with organic matter, warm water and nutrient load. Low sanitiser lets algae growth start. Poor filtration, short pump schedules and blocked baskets leave floating debris in the water so water quality drops fast. We see this pattern repeat across Perth every summer, particularly on bore water properties around Kingsley and Padbury where phosphate levels run high from garden runoff and algae feeds on the excess nutrients before anyone notices.
Top Triggers
The triggers we check first: free chlorine below target, ph above 7.8 or below 7.0, cyanuric acid out of range, a dirty pool filter or worn sand bed, poor water circulation behind ladders and ledges, and heavy rain that dilutes sanitiser. If your pool has turned green and you cannot identify the cause, a water sample will narrow it down.
How Do I Fix A Green Pool Fast
Follow this step by step guide to clear green pool water. For a detailed walkthrough, our guide on how to fix a green pool covers every scenario.
Remove Debris
Scoop leaves and floating matter from the surface and empty the skimmer box and pump basket. Remove debris from the pool floor so chemicals target algae, not plant matter.
Test A Water Sample
Check free chlorine, ph, total alkalinity, calcium hardness, cyanuric acid and phosphates. Use a reliable test kit or bring a water sample to your local pool store for a professional reading.
Balance Ph Before You Shock
Set ph levels to 7.2 to 7.6 where chlorine works hardest. Getting the water chemistry right before shocking is the step most DIY attempts skip and the reason their pool chemicals do not work.
Apply Chlorine Shock Treatment
Dose liquid chlorine at a high dose to lift free chlorine levels above 10 ppm. For a dark green pool, five times normal depending on pool size. Apply at dusk with the filtration system running to kill algae and begin the recovery.
Brush All Pool Surfaces
Use a stiff brush on pool walls, steps, lights and ladders. Brush the walls and floor to lift biofilm so chlorine can reach and remove algae underneath. On pebblecrete, aggressive brushing is needed to break the layer where green slime hides.
Run The Filtration System
Run the pool pump continuously for the first 24 to 48 hours. Backwash a sand filter when the pressure gauge rises 8 to 10 psi above clean. Hose cartridge filters top to bottom and check for tears.
Retest And Hold Chlorine
After several hours, regularly test again. Maintain free chlorine levels above 5 ppm until green turns to grey and you see dead algae settling to the floor.
Use Clarifier Or Floc If Needed
Clarifier helps tiny particles clump for capture by cartridge filters and sand filters. Use a flocculant only if you can vacuum to waste on a multiport valve. Do not floc a cartridge system unless the label specifies it is safe.
Vacuum To Remove Dead Algae
Vacuum slowly to waste if you used flocculant, otherwise vacuum on filter and backwash afterwards. Remove dead algae thoroughly so it does not feed a new bloom.
Return To Normal Targets
When the water clears, set free chlorine to 2 to 4 ppm, ph at 7.2 to 7.6, alkalinity at 80 to 120 ppm, and calcium hardness appropriate for your pool surface. For mild cases, see our guide on how to fix a green pool fast.
What Types Of Pool Algae Should I Recognise
Different algae types need different approaches. Knowing which you have helps you choose the right pool chemicals and tools.
Green Algae
The classic green pool water tint with loose growth on walls and in corners. Responds well to shock treatment, brushing and good filtration.
Mustard Algae Yellow Algae
Dusty yellow algae patches that return on the shady side of the pool surface and in shaded areas behind steps. Mustard algae resists standard doses and survives on pool equipment and pool toys. Superchlorinate, brush daily, soak removable items in strong sanitiser, and hold chlorine levels at the top of range for a full week.
Black Algae
Dark dots with a tough protective head that anchors into rough surfaces. A black algae infestation is more common on old plaster than smooth fibreglass. Aggressive brushing with a stainless brush is essential to break the head so chlorine can penetrate. We treated a pool in Duncraig with black algae embedded in old pebblecrete, and it took three rounds of targeted brushing and elevated chlorine over two weeks to clear.
Pink Algae Pink Biofilm
Slimy pink streaks in corners and behind fittings. It is a bacterial biofilm, not true algae. Heavy brushing, shock treatment and improved circulation clear it.
Which Products Help Me Fix A Green Pool
Liquid chlorine is our go to for shock dosing because it raises levels immediately without adding stabiliser. A pool algaecide supports recovery but never replaces chlorine. A phosphate remover is worth dosing after the bloom clears if phosphate levels are elevated, which they almost always are on Perth bore water. Avoid copper based algaecides if you have had staining. We carry professional grade pool chemicals at our Burns Beach shop, selected for Perth conditions.
How Do I Prevent Algae Growth Year Round
Algae prevention is where we focus most of our time with regular maintenance clients. Adrian runs a seasonal maintenance schedule on every pool we service, and the pools that follow it stay algae free through summer and winter.
Prevention Checklist
Regularly test ph and free chlorine twice a week in hot weather. Run the pool pump long enough for a full turnover each day and point returns to reach shaded areas where stagnant water collects. Backwash or clean the pool filter when pressure rises 8 to 10 psi above clean. Use a pool cover when idle to reduce UV loss and keep debris out. Rinse pool toys after an outbreak so you do not reintroduce algae spores. Keep phosphate levels modest with a phosphate remover where garden runoff is an issue. Maintain free chlorine levels in range for your cyanuric acid level. These habits keep algae at bay and keep your pool sparkling water clear year round. A pool we maintain in Greenwood has not had a single outbreak in four years on this schedule.
Does Pool Size Change How Much Chemical I Need
Yes. Dose to volume and recheck after circulation. Always calculate from length, width and average depth so you apply the correct amount.
Can Wrong Chemicals Make A Green Pool Worse
Overusing flocculant in a cartridge system clogs elements and leads to poor filtration. Copper heavy algaecides stain finishes if ph sits high. Add one product at a time with the pump running. If you are unsure about water balance, bring a water sample to your local pool store or pool shop for guidance.
Why Do Fibreglass Pools Fight Algae Differently
A fibreglass pool has a gelcoat finish where the smooth surface makes it harder for algae to anchor compared to pebblecrete or concrete. That smooth surface still needs proper chlorine levels and filtration. Keep ph and alkalinity in range to protect the gelcoat. We service several fibreglass pools in Quinns Rocks that stay pool clean with less effort because the surface does not harbour algae the way rough finishes do.
How Do Poor Circulation And Filtration Cause Algae
Poor circulation and poor filtration leave algae spores suspended in the water. Short run times, clogged baskets and worn filter media let algae settle in dead zones. Stagnant water behind ladders and around steps is where algae thrives because sanitiser does not reach it. Check that your filtration system matches pool size and the pressure gauge rises predictably between cleans. A Pentair WhisperFlo we installed in Ocean Reef solved recurring algae where the old pool pump could not turn over the 60,000 litre pool fast enough to prevent algae growth in the far corners.
When Should I Call The Local Pool Store Or A Technician
Call when chlorine levels will not hold, water balance swings between tests, or the green pool returns within days. Algae feeds on whatever your system cannot remove, and persistent blooms usually mean pool equipment or pool chemistry is failing. If your pool keeps turning green overnight or comes back green and cloudy after treatment, our team diagnoses and resolves it on site.
Frequently Asked Questions
Common questions from pool owners dealing with pool algae across Perth.
How Fast Can Algae Bloom
In hot weather with low free chlorine, algae can bloom within a few hours. We have seen pools turn green overnight after a pump fault during a heatwave.
Can I Swim In Green Water
No. Green water means sanitiser has failed. Wait until the water clears and chemical levels sit in the safe range.
Should I Leave The Cover On While I Clear Algae
Remove the pool cover during shock treatment so chlorine gas can escape and aeration is not restricted. Replace it once the water clears.
Do Saltwater Pools Avoid Algae
No. Saltwater pools still rely on chlorine generated by the cell. Set output and run time to maintain free chlorine levels and clean the cell regularly to keep production steady.
What If The Pool Turns Green Again Soon After It Clears
Recheck cyanuric acid, phosphate levels and filtration. Inspect for dead spots and adjust return eyeballs to eliminate poor circulation. A short pool algaecide program after stabilising pool chemistry can help stop algae from re-establishing. For more prevention tips, follow our green to clean pool guide.
Keep Your Pool Algae Free All Year
To prevent algae growth and stop algae from returning, maintain free chlorine levels in range, run the pump for adequate turnover, clean the filter on schedule and cover the pool when it is not in use. With consistent habits and timely checks after hot weather or storms, you can keep your pool algae free and the water safe for swimming year round.
If you want help setting up a maintenance plan or need a green pool sorted, contact our team for a green pool recovery callout. We test, diagnose and fix the problem so your swimming pool stays clear through every season.
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