Problem Summary
Pool water has turned visibly green, ranging from a light tint to fully opaque.
Green pool water is almost always algae growth, triggered when disinfection falls behind for long enough for algae to establish. Here is how to confirm the cause and correct it.
Possible Causes
Free chlorine dropped too low for too long
Algae spores are always present in outdoor water; they only take hold once active disinfectant stays below the effective range.
Poor circulation leaving dead zones
Areas with weak water movement (corners, behind steps, shaded zones) let algae establish even if bulk-water chlorine looks acceptable.
High cyanuric acid (stabilizer) locking chlorine's effectiveness
Excess stabilizer reduces how effectively free chlorine can actually kill organisms, even at a technically 'normal' reading.
Diagnostic Steps
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Test free chlorine, pH, and cyanuric acid.
Free chlorine near zero, or very high cyanuric acid, confirms disinfection has been ineffective.
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Look for algae specifically on walls, steps, and low-flow corners, not just in the open water.
Visible growth on surfaces alongside green water confirms established algae rather than a one-off color shift.
Solutions
Shock the pool with an appropriately calculated chlorine dose
Addresses cause: Free chlorine dropped too low
Use the Chlorine Calculator to work out the dose needed to bring the pool from its current level up to an effective shock level for your pool’s actual volume — never guess a dose by pool ‘size’ alone.
Brush all surfaces before and during treatment
Addresses cause: Poor circulation leaving dead zones
Brushing breaks up algae colonies so chlorine can reach them; this step is often skipped and treatment fails as a result.
Run filtration continuously until the water clears
Addresses cause: General recovery
Keep the pump running 24/7 during algae treatment rather than the normal daily schedule, until the water is clear and free chlorine holds steady.
When to Contact a Specialist
If the pool doesn’t respond after a correctly calculated shock treatment and 24–48 hours of continuous filtration, or if green water keeps recurring within days of clearing, have the circulation design and cyanuric acid level reviewed — a design or chemistry root cause may be preventing the treatment from holding.
FAQ
Can I swim while treating green pool water?
No — wait until chlorine returns to a normal, safe range and the water is visibly clear.
Does a green pool always mean algae?
Almost always, though very rarely metal content (e.g. copper) can tint water green; if chlorine and cyanuric acid both test normal and algae treatment doesn’t help, that’s worth investigating separately.