How to fix a green pool step by step, without wasting chemicals
A green pool is the most common summer emergency call. This guide covers the process professionals follow: what to test before adding anything, how to size the shock dose to the pool volume, and what to do over the following days to get the water crystal clear.
Why pool water turns green
The green colour comes from algae, which bloom when the sanitizer (chlorine, bromine or the salt chlorinator) drops below the minimum for a few days: heat waves, heavy bathing, storms or simply a missed week.
Test before you dose. A pH above 8.0 cuts chlorine's effectiveness to a fraction, and cyanuric acid above 70–80 ppm locks chlorine up even when the reading looks fine. Adding more chlorine without fixing that is money down the drain.
The professional process, in order
- 1
Test pH, chlorine and cyanuric acid
Write down real values. If pH is above 7.6, correct it first: a chlorine shock barely works at high pH.
- 2
Brush walls, floor and steps
Algae protect themselves with a slimy layer. Brushing puts them into suspension, where chlorine can actually reach them.
- 3
Apply the shock at the calculated dose
A green pool usually needs free chlorine raised to 10–15 ppm depending on severity. The exact amount depends on the pool's volume and your product: the Pool Service Hub dosage calculator gives you the grams instantly, with a safety cap and warnings if cyanuric acid is high.
- 4
Run filtration continuously for 24–48 h
The pump must run non-stop, backwashing the filter when pressure rises. The water will turn grey or milky: that's a good sign — dead algae.
- 5
Vacuum the debris and rebalance
Once the floor is visible, vacuum the sediment (to waste if the load is heavy) and bring pH and chlorine back to maintenance levels.
Shock target by water condition
| Water condition | Free chlorine target | Typical situation |
|---|---|---|
| Slightly cloudy | 5 – 8 ppm | First algae, floor still visible |
| Light green | 10 ppm | Floor barely visible, slippery walls |
| Deep green | 15 ppm | Floor not visible, several days untreated |
| Dark green / black | 15 – 20 ppm + repeat | Weeks untreated; may need two shocks |
Mistakes that slow the recovery
Shocking with high pH
At pH 8.0 chlorine loses most of its sanitising power. pH first, chlorine second.
Skipping the brushing
Without brushing, algae stay stuck to the walls and the shock only kills what floats.
Switching the pump off at night
During recovery, filtration must run continuously; every stop throws algae back into the water.
Forgetting cyanuric acid
Above 70–80 ppm of stabiliser, stabilised chlorine no longer works: you need a water change or unstabilised hypochlorite.
How to stop it happening again
The key is the reserve between visits: if the next visit is 7 days away, the water needs enough slow-dissolving sanitizer (tablets in the skimmer or chlorinator output) to arrive with chlorine to spare.
The Pool Service Hub dosage calculator does that maths for you: it compares today's readings with the last visit to estimate real daily consumption, checks the days left in the schedule and tells you how many tablets to leave or what output to set on the salt chlorinator.
Keep reading: Chemical dosage calculator · How to keep pool water clean · Pool water testing log
Frequently asked questions
How long does a green pool take to clear?+
Between 2 and 5 days depending on severity: the shock kills algae within hours, but filtration needs a day or two to remove the debris and restore clarity.
How much chlorine does a shock need?+
It depends on the pool volume and the target (10–15 ppm free chlorine). As a reference, raising 1 ppm in 10 m³ takes roughly 15–20 g of granular dichlor; the app's calculator gives the exact figure for your pool.
Can you swim after shocking?+
Not until free chlorine drops below 3–5 ppm and the water is clear. After a 15 ppm shock that usually takes several days.
What about saltwater pools?+
The process is the same, but the shock is done with liquid hypochlorite or the chlorinator's boost mode. Check salinity and the cell condition first.
Calculate the exact shock dose from the visit
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