Perth storm season runs from May through September, and after a big storm we take more calls about green pool water than any other time of year. Pool owners across the northern and southern suburbs walk outside to find the pool has turned green overnight. Heavy rain dilutes free chlorine, shifts ph levels, and washes dirt, phosphates and organic matter straight into the water. Those conditions give algae spores everything they need to take hold before anyone notices.
Adrian has been fixing post-rain green pools across Perth for over two decades, from Mandurah through to Clarkson, and we take more storm season callouts than any other job type. The pattern is consistent: the storm hits, the water chemistry collapses, and the pool turns green within hours. This guide covers why it happens, how to fix green pool water after rain, and how to prevent it from recurring. If your pool has turned green and you want it handled, our green pool recovery team can have it sorted within days.
Why Does Rain Make My Pool Green
Rain adds a surge of excess water that dilutes free chlorine below the threshold needed to suppress algae. At the same time, runoff from gardens, paths and roofs carries dirt, pollen, leaves and phosphates into the pool. That organic load consumes whatever chlorine remains. On properties around Canning Vale and Morley where garden beds sit above the pool, we see phosphate readings spike after a single downpour because fertiliser washes straight off the lawn and into the water. The combination of diluted sanitiser and a fresh nutrient load gives algae growth a head start it would not normally get.
Does Rainwater Cause Algae In Pools
Rainwater itself does not contain live algae in any meaningful quantity. What it does is create conditions where algae spores already sitting in every pool can multiply unchecked. Reduced chlorine levels, shifted ph, and an influx of nutrients from garden runoff remove the barriers that normally keep those spores dormant. We tested a pool in Mindarie after a winter storm where free chlorine had dropped from 3 ppm to 0.4 ppm in a matter of hours. The pool had a light green tint by morning and visible algae on the shaded wall by that afternoon.
What Happens To Pool Water Chemistry After Heavy Rain
Heavy rain pushes ph levels down because rainwater is slightly acidic, typically around 5.5 to 6.0. Total alkalinity drops as the excess water dilutes the buffer. Free chlorine falls because the volume increases and the organic matter washed in consumes it. Calcium hardness usually holds unless you receive a large volume of soft rainwater over several days. With the pool’s alkalinity lowered, ph becomes unstable and swings more easily, which makes chlorine less effective at killing algae. Test the water as soon as the rain eases so you can correct chemical levels before things get worse.
Can Rain Make Pool Chlorine Less Effective
Yes. Even if your chlorine levels read within normal levels before the storm, rain dilutes both the chlorine and the stabiliser that protects it. Once cyanuric acid drops, the sun strips chlorine out within hours on a sunny day after the weather clears. We serviced a pool in Woodvale where the owner had shocked the night before a storm. By the following afternoon the free chlorine read zero and the water had a green tint. The rain had diluted the stabiliser so severely that UV burned through the chlorine faster than the Astral VX salt cell could replace it.
How Long After Rain Does A Pool Turn Green
Most pools show a green tint within 12 to 48 hours if the pump stays off and chemical levels sit out of range. Pools with a pool cover, strong circulation, and balanced chemicals often handle light rain without any issues. We find the pools that turn green fastest are the ones where chlorine was already borderline before the storm hit. A big storm on a pool with marginal chemistry is almost always a green pool by the next morning.
Is Green Pool Water Dangerous To Swim In
Do not swim in green water. It means sanitiser has failed and harmful bacteria can multiply alongside the algae. Other contaminants washed in by rain, including garden chemicals and animal waste, compound the risk. Poor visibility at the floor and across pool surfaces means you cannot see hazards below. Eye irritation is common even in mildly green water. Get the pool treated before anyone goes swimming.
Why Does My Pool Always Go Cloudy Or Green After Storms
If your pool turn green after every storm, the issue runs deeper than just the rain. It usually points to pool equipment that is not working properly or regular maintenance that has fallen behind. Short pump run times, a clogged skimmer box, worn pool filter media, or a salt cell producing below capacity all reduce your pool’s ability to recover from a chemical hit. We diagnosed a pool in Currambine that went green and cloudy after every winter storm. The Waterco sand filter had not been serviced in three years and the media was channelled so badly it was passing particles straight through. Once we replaced it, the pool handled two consecutive storms without turning green. If your pool keeps turning green despite treating it, the equipment needs checking.
Do Saltwater Pools Turn Green After Rain Too
A saltwater pool is not immune. The chlorinator only generates chlorine when the pump runs, and heavy rain dilutes salt concentration alongside chemistry. If salt drops below the manufacturer’s instructions threshold, the cell reduces output or shuts down entirely. After storms, check salt levels, boost chlorinator output or manually shock the pool, and run longer pump cycles until the water clears. We add salt on post-rain callouts more often than any other time of year.
How Do I Prevent A Pool From Going Green When It Rains
Preventing algae after rain comes down to having chemistry and equipment in strong shape before the storm arrives. Keep free chlorine at proper levels between 2 and 4 ppm and ph between 7.2 and 7.6 year round. Run the pump for eight to twelve hours a day in cooler months and increase run time during storm season. Fit a pool cover when possible to reduce dilution and keep large debris out. Trim overhanging branches and keep garden beds tidy so phosphate runoff stays minimal. Empty skimmer baskets before the storm hits. Service pool equipment at the start of each season to confirm everything is working properly.
A pool in Butler we maintain has not turned green through three consecutive storm seasons because the owners follow this routine and we check the balance quarterly. These habits give your pool the resilience to handle heavy rain without losing clarity.
How Do I Fix A Green Pool After Rain
Follow these steps in order to fix green pool water and get back to crystal clear swimming conditions. Speed matters because the longer algae sits, the harder pool cleaning becomes and the greater the risk of costly repairs to your filter system.
Remove Large Debris
Net out leaves, seed pods and other debris from the surface and floor so chlorine targets algae, not plant matter. Empty the skimmer box and pump baskets. Remove large debris first to stop it breaking down and feeding the algae further.
Test And Balance Water Chemistry
Use a reliable test kit or bring a water sample in for free water testing. Check ph, total alkalinity, free chlorine, calcium hardness and stabiliser. Bring ph to 7.2 to 7.6 and alkalinity levels to 80 to 120 ppm before adding anything else. If ph is low, dose soda ash in the correct amount per your pool volume. Getting the water chemistry right first means your shock treatment will actually work.
Shock The Pool
Dose liquid chlorine at a high dose to lift free chlorine above 10 ppm. For a light green tint, double the normal dose. For dark green water where you cannot see the floor, we dose at five times normal following manufacturer’s instructions for the pool size. Apply at dusk so UV does not strip it overnight. This is the step that kills live algae and begins the recovery.
Brush Walls And Floor
Brush pool walls, pool surfaces, steps and the pool floor with a stiff bristle brush to break up biofilm. Algae clings to pebblecrete and grout lines where pool chemicals cannot penetrate without physical agitation. Focus on shaded corners and under return jets where circulation is weakest.
Run And Clean The Filter
Run the pump and filtration system to filter continuously for 24 to 48 hours. The filter traps dead algae and suspended particles as the water clears. On a sand filter, backwash when filter pressure rises 8 to 10 psi above the clean reading. On a cartridge filter, hose the element when flow drops. We cleaned a filter three times on a post-storm recovery in Mandurah before the water shifted from green to clear. Keep the pump running until clarity returns.
Use Clarifier Or Floc Only When Needed
If the water is still hazy after dead algae has been filtered, a clarifier helps fine particles clump for the filter to trap. On a sand filter, a flocculant drops particles to the floor for vacuuming to waste. Do not use flocculant on a cartridge filter system as it clogs the pleats and can cause costly repairs to the element.
Treat Nutrients If Algae Returns Quickly
If green pool water keeps returning after rain, test phosphates. Garden runoff is the most common source across Perth. A phosphate remover cuts the food supply and makes preventing algae far easier long term. On bore water properties we dose a phosphate remover as part of every post-storm treatment because the levels are consistently elevated.
Retest And Fine Tune
Retest after 24 hours. If free chlorine holds above 5 ppm and the green is fading, the shock treatment is working. If chlorine levels are still dropping overnight, there is residual algae and you need another round. Once the pool clears, return chlorine to normal levels of 2 to 4 ppm and confirm all chemical levels are stable. For the full step by step recovery process, follow our green to clean pool guide.
What Setting Should I Use On My Filter After Rain
Run the filter on its normal setting and backwash or clean when filter pressure rises 8 to 10 psi or flow slows noticeably. On a sand filter, backwash until the sight glass runs clear then rinse before returning to filter mode. On a cartridge, pull the element and hose from top to bottom. Vacuum to waste only when heavy silt has settled on the floor and you can afford to lower the water level.
Will An Algaecide Help After Storms
Adding algaecide can support recovery but correct chlorine levels do the heavy lifting. An algaecide works best as a preventive measure dosed before storm season rather than a fix after the fact. Avoid copper based formulas if you have had staining or if your bore water already carries elevated metals. Follow the manufacturer’s instructions on dosage and do not mix chemicals in the same dose.
What Does A Professional Do Differently
We measure exact chemical demand rather than guessing dosages. Adrian checks pump output, filter media condition, chlorinator performance, and circulation across every zone of the pool. On a post-rain callout in Clarkson last winter, we found the Davey SureFlo pump had lost prime during the storm and run dry for six hours before the owner noticed. The pool was green, but the real issue was the pump seal. Fixing both the seal and the chemistry meant the pool recovered properly and stayed pool algae free through the rest of winter. For a deeper look at the full how to fix a green pool process, that guide walks through every step we follow on a recovery job.
Quick Answers To Common Questions
Common questions we hear from Perth pool owners after storms and heavy rain.
Can Dirt And Debris From Rain Cause Algae
Yes. Organic matter feeds microorganisms and shields algae from chlorine. Remove large debris and other debris quickly after any storm to reduce the load on your chemicals and filter.
Do Cartridge Filters Or Sand Filters Clear Greens Faster
Both work when clean and correctly sized. Cartridge captures finer particles but must be cleaned thoroughly during recovery. Sand needs correct media depth and regular backwashing to maintain flow and performance.
How Long Until Water Clears After Rain
With correct balance, a proper shock treatment and the pump running to filter continuously, most pools clear within 24 to 48 hours. Severely green pools where the filtration system is compromised can take three to five days.
Do Saltwater Pools Need Shocking
Yes. A saltwater pool chlorinator is not designed to lift chlorine by large amounts quickly. Shock manually with liquid chlorine after storms, then let the cell maintain proper levels once the water clears.
Is Green Water Always Algae
Not always. Dissolved copper from bore water or old heat exchangers can tint pool water green without any algae present. If the water stays green after correct chlorine and filtration, test for metals and treat with a sequestrant.
Prevention Checklist For The Next Storm
Keep ph, alkalinity levels, calcium hardness, and chlorine at proper levels before the weather hits. Fit and use a pool cover to reduce dilution and debris. Run longer pump cycles during and after heavy rain. Empty skimmer baskets when full. Keep gardens tidy to minimise phosphate wash in. Service pool equipment before swimming season to confirm everything is working properly. Dose a phosphate remover if you are on bore water or have garden beds above the pool line. These steps keep your pool resilient through storm season and reduce the chance of waking up to turned green water after every downpour.
When Should You Call For Help
If your pool has turned green after rain and stays green despite correct treatment, or if the green keeps returning after every storm, there is likely an equipment or circulation problem that chemicals alone cannot solve. Persistent green pool water after rain often signals undersized pump run times, failing filter media, or a chlorinator cell near the end of its life. Do not let it drag on, because the longer a pool sits green the more it costs in chemicals and potential damage.
Contact our team for a green pool recovery callout. We test the water, inspect the equipment, and work out exactly why the storm hit your pool harder than it should have. From a light green tint after a shower to dark green water after a week of rain, we have seen and fixed it all across Perth. Your swimming pool does not have to turn green every time it rains.
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