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 haboob does not announce itself. It arrives on an ordinary evening, drops more debris into the water in twenty minutes than a quiet month does, and leaves behind silt on the floor, a spiked chemical demand and a filter working against a load nobody has checked.
Evaporation on a scale that changes the chemistry. A quarter-inch a day off the surface in summer is normal. The water leaves; its calcium stays, and the hose that replaces it brings more. That is why scale and hardness 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 602-313-8963.
