Green pool water means algae growth and is not safe to swim in. Algae allow bacteria to thrive and chlorine stops working effectively. The treatment: shock dose to 10 to 30 mg/l free chlorine, algicide, daily backwashing and patience. In most cases the water is clear again within 3 to 5 days.
Is green pool water dangerous?
Yes. Green pool water is unsafe to swim in. There are two distinct risks:
Bacteria: algae themselves are not harmful, but they create conditions where pathogens like E. coli, Pseudomonas and Legionella can multiply. In water with no active chlorine, these bacteria can reach harmful concentrations within 24 hours.
No disinfection: when water is green, free chlorine is almost always at or near zero. The water offers no protection against bacteria and viruses introduced by swimmers (sweat, urine, skin cells).
Do not swim in green pool water. Wait until the water is clear, free chlorine is between 1.0 and 3.0 mg/l, and pH is between 7.2 and 7.6.
Why does algae turn water green?
Algae are microscopic plant organisms that produce chlorophyll, the pigment that gives them their green colour. They multiply rapidly when:
- Free chlorine drops below 1.0 mg/l
- pH is too high (chlorine works less effectively)
- The filter pump runs too few hours per day
- Water temperature rises above 25 degrees in warm, sunny weather
Algae grow on the walls, the floor and suspended in the water itself. Slimy walls are an early sign of algae growth, often visible before the water colour changes.
Types of algae in pools
Green algae (most common): colour the water light green to deep green. Float in the water or cling to walls.
Yellow or mustard algae: attach to the walls, harder to treat. Look like sand or dust on the wall surface.
Black algae: deep blue-black spots on walls, the hardest type to remove. They form a protective outer layer and require intensive treatment including wire brushing.
How to fix green pool water: step-by-step
Step 1: Test the water
Measure pH, free chlorine and alkalinity. With green water, the chlorine reading is almost always zero or near zero.
Step 2: Correct the pH
Bring pH to 7.2. At a slightly lower pH chlorine is most effective for the shock treatment.
Step 3: Shock the pool
Add liquid chlorine or calcium hypochlorite granules:
- Light green water: target 10 mg/l free chlorine
- Dark green water: target 20 to 30 mg/l free chlorine
Dosage for calcium hypochlorite (70%): approximately 140 grams per 10,000 litres to achieve 10 mg/l.
Always shock in the evening. Daytime UV destroys the extra chlorine too quickly to be effective. Run the pump at full capacity throughout the night.
For the full shock treatment procedure, read our pool shock treatment guide .
Step 4: Add algicide
After the shock treatment (wait at least 4 hours), add a copper-based algicide according to the dosage on the label. Algicide is not a disinfectant: it helps the dead algae clump together so the filter can remove them.
Warning
Never add algicide at the same time as chlorine. Chlorine oxidises the algicide and makes it less effective. Always wait at least 4 hours after shocking.
Step 5: Brush walls and floor
Use a pool brush on a telescopic pole. Brush all walls, the floor and especially corners and steps. This brings algae into suspension where the chlorine can reach them more effectively.
Step 6: Backwash daily
Backwash the sand filter daily for 3 to 5 minutes. The filter clogs quickly with dead algae. Without daily backwashing the filter loses capacity and the water stays cloudy.
Step 7: Run the pump 24 hours
Run the pump continuously until the water is clear. Normal operation is 8 to 12 hours per day, but during algae treatment 24-hour circulation is necessary.
Step 8: Assess after 24 hours
- Water is lightening in colour: treatment is working
- Water remains dark green: repeat the shock dose
- Water is blue-grey and cloudy: this is normal, the algae are dead but still in suspension. Wait for filtration to remove them.
How to prevent green pool water
Prevention is always cheaper than treatment. A pool that has turned fully green costs 2 to 5 times more effort to restore than weekly preventive maintenance.
- Keep free chlorine above 1.0 mg/l at all times; test 2 to 3 times per week
- Check pH weekly: above 7.6 chlorine loses effectiveness rapidly
- Add preventive algicide weekly during warm, sunny periods (above 25 degrees)
- Run the pump at least 8 hours per day, 12 hours when temperatures exceed 25 degrees
- Brush walls and floor weekly to prevent early algae attachment
- Cover the pool at night and during rain (rain lowers pH and dilutes chlorine)
- After a party or heavy use, always run a preventive shock dose
Read more about ideal chlorine levels and when to top up in our pool chlorine guide . For the role pH plays in algae prevention, see pool pH: how it affects chlorine .
Preventive Algicide (1 litre)
BayrolPreventive algicide stops green algae before it starts. Add 50 ml per 25,000 litres weekly. Compatible with all filter types.
- Effective prevention
- Non-foaming
- Compatible with all filters
- Does not treat existing algae outbreaks
Why algae keep coming back
Recurring algae is almost always a system problem, not a one-off event. Three root causes account for the majority of repeat outbreaks.
Insufficient chlorine residual: chlorine demand exceeds supply. This happens when cyanuric acid (CYA) has accumulated above 80 mg/l, locking chlorine into an inactive form. Test CYA once per month and partial-drain the pool if it exceeds 80 mg/l. All those weeks of 200g trichlor tablets have added roughly 6 grams of CYA each to the water.
Inadequate filter run time: a pump running 6 hours per day turns over a 20,000-litre pool just 1.8 times. For every degree the water temperature rises above 25 degrees, algae growth rate doubles. At 30 degrees, run the pump 12 to 14 hours.
Undersized filtration: if your pump is rated 4 m3/h for a 50,000-litre pool, the turnover time is 12.5 hours. Even 24-hour operation gives fewer than 2 turnovers per day. Upgrade the pump or accept that you will always be fighting chemistry with an inadequate physical barrier.
Cloudy blue-grey water after treatment
After a successful shock treatment, the water often turns from dark green to a milky blue-grey. This is not failure. The green algae are dead but the dead cells are still suspended in the water and the filter is slowly removing them.
Keep the pump running 24 hours per day. Backwash the filter every 12 hours. Within 24 to 48 hours of continuous filtration, the water should be visibly clearer. If it remains hazy after 48 hours, add a dose of liquid flocculant (sand filter only) to clump the fine particles for faster removal.
If the water returns to green again within 24 hours of the shock, the chlorine demand is higher than the dose you applied. Repeat the shock at the higher end of the range (30 mg/l) and check whether your CYA level is contributing to chlorine lock.
Algae on the walls vs. in the water
Green floating algae in the water respond quickly to shock and filtration. Wall algae (slimy coating) are more persistent because they are partially protected by the biofilm they produce. Brushing brings them into suspension so the chlorine can reach them.
Black algae form a multi-layered structure with a waxy protective outer coating. Standard shock treatment at 10 mg/l is often not enough. For black algae, target 20 to 30 mg/l free chlorine and use a stainless steel brush to break through the protective layer before applying the dose. Repeat the treatment every 48 hours until no new black spots appear.
Green algae on pool walls: how to remove
Green deposits on pool walls are a separate situation from green water. These are algae that have attached to the surface, tiles, liner, concrete or fibreglass, forming a slimy green layer, sometimes called biofilm.
Green water vs. green wall deposits:
- Green water: algae floating in suspension: treat with shock chlorination
- Green wall deposits: algae attached to the surface: requires physical brushing plus chlorine
How to remove green algae from pool walls:
- Brush off the deposits with a stiff pool brush (nylon for liners, stainless steel for concrete). Work from top to bottom.
- Apply a shock treatment (10 to 20 mg/l) to kill the loosened algae.
- Backwash the filter after 24 hours to remove the dead algae.
- Repeat brushing and backwash after 48 hours if deposits remain.
For persistent green deposits on tiles or concrete, you can apply a concentrated algicide solution directly to the surface with a sponge: switch the pump off first, work quickly.
For severe algae growth and the correct shock dosage, read our pool shock treatment guide .
