A green pool almost always means algae growth from low free chlorine, warm water and strong sun, plus extra organic load from dust, leaves and sunscreen. When chlorine levels fall below what your cyanuric acid allows, algae takes hold on pool surfaces and in the pool water. Poor circulation or a clogged filtration system shortens contact time, so the sanitiser cannot kill algae fast enough.
What causes a pool to turn green
Low free chlorine lets algae bloom. Heat, sunlight and heavy bather load raise demand, so a normal dose no longer maintains balance. Dust, leaves and oils add nutrients that consume oxidiser and leave floating debris. A dirty pool filter or undersized pool pump reduces turnover and allows an algae outbreak to spread.
Is green water always algae, or can it be metals like copper or iron
Most green pool water is algae, but metals can tint clear water green too.
- Algae: water looks cloudy green, pool walls feel slippery, and clarity worsens after sun.
- Metals: water can look clear green after a shock treatment and you often see brown or black stain marks.
Confirm the cause before dosing. If your test shows good free chlorine but the pool turns green, read How To Fix A Green Pool
Can high phosphates make my pool turn green
Phosphates feed algae but do not cause blooms if free chlorine stays in range. Fix sanitiser first. Improve circulation and filtration, then treat high phosphates if a test confirms a concentrated level.
Does low chlorine or high combined chlorine cause green water
Low FC enables growth. High CC signals chloramine buildup and poor oxidation. If CC is high, shock correctly, raise circulation, and clean or backwash the filter so the water flow is strong enough to push oxidiser through the whole system.
How do pH and alkalinity affect algae growth
High pH weakens chlorine strength; aim 7.2 to 7.6 during recovery. Stable total alkalinity limits drift so the residual stays effective. That chemical balance helps you kill algae and hold clear water.
Can rainstorms or pollen turn a pool green
Yes. Rain dilutes pool chemicals and adds dust. Pollen increases fines that cloud water. Skim and pool brush first, then shock at dusk if FC collapsed, and extend run time for 24 to 48 hours. To reduce debris input between storms, compare fitted options in Discover the Best Pool Cover Australia Has to Offer
How fast can algae grow in warm weather
Very fast. In Perth heat and sun, green algae can double in a day when FC is low and the filter is dirty. Test on a regular basis during warmer months and whenever the pool has heavy use.
What are the types of algae and how do I tell
Identify the type so you choose the right fix.
- Green algae: free floating and on walls. Clears quickest with shock plus firm brushing.
- Yellow or mustard algae: clings to shaded walls, ladders and the pool floor. Needs a higher shock level and meticulous brushing.
- Black algae: pinhead dots that root into plaster. Requires repeated oxidiser, strong brushing and patience.
For a full overview, see Things You Need to Know About Pool Algae
Is cloudy green water different from clear green water
Yes, and the treatment is different.
- Cloudy green water: active bloom plus suspended particles. Prioritise shock and filtration.
- Clear green tint: often metals after a shock. Confirm with a test before you add sequestrant or partially refill.
Can a clogged or undersized filter make a pool turn green
Absolutely. Low flow and trapped debris reduce sanitiser contact time. Backwash a pool with sand or glass media when the pressure gauge rises 20 to 25 percent above the clean reading, or hose cartridge filters until the pleats are clean. If you are deciding on media and maintenance, read Pool Sand or Cartridge Filter: Which Is Better?
How does cyanuric acid affect chlorine’s ability to kill algae
Higher CYA requires a higher free chlorine target for the same kill rate. Very high CYA makes recovery slower. When CYA is excessive, a partial drain and refill is the practical fix. Learn how stabiliser works here: Cyanuric Acid
Can saltwater pools turn green too
Yes. Low salt, scaled cells or short run times reduce chlorine production, so a salt pool can also turn green. Shock with liquid chlorine while you correct salinity and clean the cell. Step by step guidance is in How to Add Salt to Your Pool
What testing confirms what is causing the green colour
Use a drop kit for FC, CC, pH, TA and CYA, and add a metals check if you see stains. Log a water sample morning and evening until the pool clears. A solid routine lives here: Testing Pool Water
Do leaves and organics in the pool fuel algae growth
Yes. Leaves, dust and sunscreen increase demand and slow recovery. Remove debris promptly and consider a robot to keep the floor clean between vacuums. If you want to understand the options and the cleaning process, start with How Do Pool Cleaners Work?
Can sunscreen and bather load contribute to green water
They can. Oils raise chlorine demand and cloud water after a pool party. Plan a light shock and longer filtration window after big swims. For simple weekly habits that prevent a re occurring problem, see How Often Should a Pool Be Serviced?
What to do next: fast recovery steps
Balance pH, brush hard, shock, and run the filter until the water turns crystal clear.
- Lower pH to about 7.2 to 7.4. Chlorine works harder at this level.
- Brush walls, steps and the floor. Break the biofilm so oxidiser can reach spores rooted in rough areas.
- Shock treatment in the evening. Use the correct dose for your CYA. Hold shock level with frequent testing for 24 to 48 hours.
- Run the pump continuously. Keep the filter running to trap dead algae and fines.
- Backwash or clean when pressure rises. Backwash sand or glass at a 20 to 25 percent rise. Hose cartridge filters thoroughly.
- Vacuum the settled debris. Vacuum to waste if the floor is covered with silt so you remove it from the system.
- Re test and balance. Restore FC to daily range, confirm pH and alkalinity, and top up calcium and stabiliser if water loss was large.
For a complete walk-through, follow How to Fix a Green Pool
Frequently asked questions
Prevention checklist
- Keep free chlorine in range for your CYA. This single habit prevents almost every algae problem.
- Run adequate turnover each day. Longer, lower RPM cycles often give better clarity with less noise and cost.
- Brush weekly. A five minute brush prevents biofilm and keeps pool walls clean.
- Empty baskets often. A clear skimmer box and pump basket protect flow.
- Test more in heat or after big swims. Warm water and sunlight demand more oxidiser.
- Maintain the filter. Backwash or clean as soon as the gauge rises 20 to 25 percent.
- Control debris. Use a cover in leaf season and net other debris promptly.
Bottom line
a green pool is a chemistry and circulation problem that you can fix with the right order of operations. Kill algae with adequate free chlorine, run the filter valve in the correct settings, and remove debris so sanitiser is not wasted. Keep a simple log, test consistently, and your swimming pool returns to crystal clear water without guesswork.
With over 20 years of industry experience, Adrian Mole is the founder of 1 Pool Care, a leading mobile pool service in Perth. Known for his expert knowledge and reliable service, Adrian delivers professional pool cleaning, equipment repairs, and water balancing across the metro area. Backed by SPASA accreditation, he’s committed to quality, convenience, and customer satisfaction.
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