Pool owners are often shocked by the speed. The pool was fine on Saturday and by Wednesday it is a pond. In Phoenix summer conditions, four days is completely ordinary — and the process started well before the colour changed.
The sequence
Day 0 — chlorine hits zero. Something consumed it: a hot week, a pool party, a storm, or simply nobody adding any. Sanitiser is now gone.
Day 1 — colonisation. Algae spores are already in every outdoor pool; they arrive on wind, rain and swimsuits. With nothing to kill them they attach to walls, steps and grout lines. The water still looks clear.
Day 2 — the water dulls. Not green yet, just slightly less sharp. This is the last cheap moment to intervene.
Day 3 — visible bloom. Free-floating algae multiplies exponentially. The water goes hazy green.
Day 4 — full green. The bottom disappears. Now it is a recovery job, not a chemistry adjustment.
What sets it off in Maricopa County
Monsoon season, roughly mid-June through September. A single haboob can drop more debris into a pool in twenty minutes than a month of ordinary weather. Dust settles into the water, raises demand, clouds the filter and buries the bottom in silt — and it happens on a random Tuesday evening with no warning.
Extreme evaporation. A Valley pool can lose a quarter-inch a day in summer. Every gallon that evaporates leaves its calcium behind and every gallon of fill water brings more in, which is why hardness and scale are the defining chemistry problem in Phoenix rather than an occasional nuisance.
Phosphates. Decaying leaves, pollen, fertiliser runoff and blown-in dirt all add phosphate, which is algae food. High phosphate does not cause a bloom on its own, but it makes one far harder to stop once chlorine slips.
Stabilizer that is too high. Cyanuric acid protects chlorine from the sun, but above roughly 80–100 ppm it locks chlorine up. The test kit reads chlorine present while the pool behaves as if there is none — a genuinely common and genuinely confusing failure here.
Stopping it instead of fixing it
The cheap version of this problem is a weekly visit where somebody tests the water, corrects it and brushes the surfaces algae would otherwise colonise. The expensive version is a recovery.
If your pool has gone green, it can almost always be brought back without draining. See how a green pool recovery works.
Would rather not do this yourself? Our complete weekly service in Phoenix starts at $124 a month with routine chemicals included and no contract. Get a free quote or call 480-448-2455.
