Real-life case: "I shocked it and the water is still green"
- The situation: two successive shock treatments, water still green 3 days later — the classic frustration.
- The 3 real causes: underdosed shock, pH too high at the time of shocking, saturated stabiliser that locks up the chlorine.
- Here: all three at once — eyeballed dose, pH at 8.0, very high stabiliser.
- Full diagnosis → shock recalculated at the right pH → clear water in 48 h.
“This can’t be right, I put in TWO shocks!” The message comes up again and again on every pool forum, and it’s the most maddening frustration an owner can feel: you did what you thought was the heavy hitter — the shock treatment — and the water stays hopelessly green. Two buckets, sixty euros of product, three days of waiting, zero result. Here’s why a shock “fails to take”, on a real case where all three culprits had ganged up.
The myth of the “magic shock”
In the collective imagination, shock chlorine is a red button: you press it, the water turns blue again. The reality is more demanding: a shock only works if three conditions are met at the same time. When one is missing, the water stays green despite the effort — and you blame the product, the time, the weather, anything but the real reasons. This pool had all three.
The full test said it all. pH at 8.0 (far too high), very high stabiliser (saturated by three seasons of tablets), and — going back through the history — shock doses estimated “by the bucketful from the forum”, with no connection to the real 55 m³. Three mistakes, one single consequence: chlorine present on the test but almost inert in the water.
Why each factor kills the shock
pH too high. This is saboteur number one. At pH 8.0, most of the chlorine you pour in goes into its inactive form: you can add kilos of it, it works at 20% of its power. It’s the step everyone skips and the one that ruins the most shocks.
Saturated stabiliser. At high concentration, it “locks up” the chlorine: the test shows a nice figure, the actual sanitisation is flat on its back. Worse, every shock with stabilised chlorine tightens the cage further.
The eyeballed dose. Green water demands a precise concentration, calculated on the volume and the starting point. “A bucket because it looks like a lot” is, mathematically, the recipe for the underdose that fades the algae without killing it.
Putting things back in order
The plan took the causes one by one. First the pH, brought back towards 7.2 in calculated steps. Then the shock, recalculated on the real 55 m³ and with non-stabilised chlorine (so as not to worsen the saturation). Continuous filtration started. Result at 24 h: the water finally turned milky grey — the sign the algae were dying for good this time. Clear water at 48 h. And the saturated stabiliser added to the list of late-season jobs (partial water renewal, the only real solution).
What this case teaches
A shock that doesn’t work is never fate, nor a matter of patience: it’s a parameter that’s blocking it, and there are only three on the list. Before buying another bucket, test the pH and the stabiliser — the answer is nearly always there. That’s exactly the app’s reflex: it refuses to have you shock water at pH 8.0, corrects the order of the steps, and calculates the right amount for YOUR pool. The right shock first time costs less than three wrong ones.
Frequently asked questions
Why is my shock chlorine doing nothing to the green water?
Three causes, nearly always: the dose is too low (generic doses from the internet ignore your volume and your starting level), the pH is too high (above 7.6 chlorine loses most of its strength), or the stabiliser is saturated and locks up the chlorine. Often all three combine.
How long does a shock take to act on green water?
Properly dosed and at the right pH, you see the water turn from green to grey-white in 24 h, then clear up over another 24-48 h with continuous filtration. If nothing moves at 24 h, it's not a matter of patience: a parameter is blocking it.
Should I double the dose if the first shock didn't work?
Not blindly. Re-test first: if the pH was too high or the stabiliser saturated, doubling the dose at the wrong pH will just waste product. Fix the cause, then dose correctly.