You have dosed the pool, the test strip reads high chlorine, and the water is still green. This is one of the most frustrating pool problems there is, and it almost always comes down to one thing: the chlorine is present but it is not working.
Why your pool is green but chlorine is high
Chlorine that reads high on a test can still be chemically useless. There are four common reasons, and you can usually identify yours in ten minutes.
High stabiliser is locking up your chlorine
The most common cause we find. Cyanuric acid protects chlorine from UV, but above roughly 75ppm it locks the chlorine so tightly that it stops sanitising. Your test shows chlorine because it is there. It just cannot do its job. This builds up silently in pools dosed with stabilised granules season after season. More detail in pool stabiliser and CYA levels.
High pH is crippling chlorine effectiveness
At pH 8.0 chlorine is a fraction as effective as it is at 7.2. Same chlorine reading, far less killing power. Cape Town pools drift high on pH easily, especially after rain or heavy topping up.
You are reading total chlorine, not free chlorine
Cheap strips often report total chlorine, which includes the spent, combined chlorine that no longer sanitises. A pool can read 5ppm total while having almost no free chlorine at all. Test free chlorine specifically.
The green is metal, not algae
If the water is clear green rather than cloudy green, and it went green right after a shock dose or a top-up from a borehole, you are probably looking at oxidised copper or iron rather than algae. Adding more chlorine makes it worse, not better.
How to fix a green pool when chlorine is already high
- Test properly. Free chlorine, pH and cyanuric acid, with a drop test rather than old strips
- Correct pH first. Bring it to 7.2 to 7.4 before dosing anything else
- Check stabiliser. Above about 75ppm the only real fix is partial draining and refilling to dilute it
- Brush and vacuum. Algae clinging to walls is shielded from chlorine
- Check circulation. A blinded filter means the water is not actually being treated, no matter what you pour in
- Run the pump longer. Circulation is what carries chemicals to the algae
When a green pool is a filtration fault, not a chemical one
We are called to plenty of green pools where the chemistry was fine and the filter had quietly stopped filtering. Matted or channelled sand lets water pass without cleaning it, and the pressure gauge can still look normal. If your pool goes green repeatedly despite correct dosing, the problem is mechanical. See sand filters explained and when to replace filter sand.
Green pool with high chlorine: frequently asked questions
Why is my pool green but the chlorine is high?
Usually because the chlorine is present but not active. High cyanuric acid above roughly 75ppm locks chlorine up, high pH above 7.8 cripples its effectiveness, or your test is reading total chlorine rather than free chlorine. Clear green water can also be oxidised metal rather than algae.
How do I clear a green pool with high chlorine?
Correct pH to 7.2 to 7.4 first, test cyanuric acid and dilute by partial drain if it is above 75ppm, brush the walls and steps, vacuum the floor, and make sure the filter is actually filtering. Adding more chlorine on top of locked-up chlorine achieves nothing.
Can too much chlorine turn a pool green?
Indirectly, yes. A heavy shock dose can oxidise dissolved copper or iron and tint the water green. That green is clear rather than cloudy, and more chlorine makes it worse.
Is it safe to swim in green pool water?
No. Green water means either uncontrolled algae, which harbours bacteria and hides the pool floor, or dissolved metals. Neither is worth swimming in. Fix the water first.
How long does it take to clear a green pool?
With correct chemistry and a healthy filter, 24 to 72 hours for most pools. If the filter is blinded or the stabiliser is locked high, it will not clear at all until that is fixed.
What is the ideal cyanuric acid level for a pool?
Around 30 to 50ppm gives UV protection without locking chlorine. Above 75ppm chlorine effectiveness falls off sharply and the practical fix is dilution.
Why does my pool keep turning green?
Repeated greening despite correct dosing points to a mechanical fault rather than chemistry. The usual culprits are a matted filter bed, a worn pump, or running the pump too few hours a day for the pool volume.
Still green? Get the circulation checked
If you have corrected the chemistry and it is still green, the fault is mechanical. See clearing a green pool fast or book a check on our pool repairs page.
Call / WhatsApp a photo of your pool for a free quote: 069 955 1310
Email: info@gibbspools.co.za
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