WATER QUALITY
Clearing green pool water
Green water is algae that took hold while the chlorine sat too low, so brush the pool, shock it and run the pump non-stop — it clears in 24 to 48 hours.
Algae can take hold within hours once free chlorine drops below 1 ppm. In Thailand that usually follows heavy rain, which dilutes the chlorine and washes dust in at the same time; after that, a run of very hot days, or a filter timer set too short. Green water does not mean the pool is damaged. It means the chlorine ran out for a while.
Do it in this order
Two things decide whether the shock works: the pH when you add it, and the time of day. Chlorine kills algae hardest at pH 7.2–7.4, and above that band less than half of what you add does any work. Shocking in daylight loses more than half the dose to UV, which is why this is always an after-dark job. By the next morning the water turns from green to a grey haze — that is dead algae, not the pool getting worse.
Net out the leaves and coarse debris. Algae clings to them.
Brush the walls and floor thoroughly — the corners, under the steps and around the returns.
Test the pH and bring it to 7.2–7.4 before you shock.
Backwash the filter before you start.
After sunset, measure out 15–20 g of 65% granular chlorine for every cubic metre of water in the pool.
Dissolve the whole dose in a bucket of water. Never scatter it in dry.
With the pump running, pour it slowly all the way round the pool.
Run the pump for 24 hours without stopping. Do not switch it off overnight — this is the step people skip most often.
The next morning, vacuum the floor straight to waste rather than through the filter.
Top the pool back up to the middle of the skimmer mouth.
Backwash the filter once more.
Bring the water back to normal: pH 7.2–7.6 and free chlorine 1–3 ppm.
Set the filter timer to 8–10 hours a day.
If it keeps coming back
Cyanuric acid (stabiliser) above 50 ppm. Your test kit still reads chlorine, but that chlorine cannot sanitise. The fix is to dilute with fresh water, not to add more chlorine.
Under 8 hours of filtration a day. In this climate the pool needs 8–10 hours, which can be split into a morning and an evening run.
Filter sand more than 5 years old. Once it channels and clumps, water runs through the gaps and is barely filtered at all.
A dead spot where water does not circulate — under the steps, a corner the returns do not reach, or beneath a cover left closed for days. Algae restarts from the same place every time.
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