Every pool owner meets the green pool eventually. You look out the window on a Tuesday and the water is fine; by Saturday morning it looks like a pond. It feels sudden. It isn't. The bloom you see on Saturday started chemically days earlier — you just couldn't see it yet.
Understanding that timeline is the difference between a five-day fix and a month of frustration. Here's what's actually happening in the water, and the exact sequence we use to bring a pool back.
What 'overnight' really means
Algae doubles fast in warm, sunny, under-sanitized water — but it's invisible until the colony reaches millions of cells per liter. The trigger is almost always the same chain: chlorine drops below its working minimum (usually after a heat wave, a heavy swim weekend, or a storm diluting the water), and the algae that every pool carries in trace amounts gets a 48-hour head start.
Two accelerants make it worse. Phosphates — from fertilizer drift, leaf litter, even some fill water — are algae food. And a clogged or channeling filter stops removing what the chlorine kills, so dead algae recirculates and clouds the water even as sanitation recovers.
The 5-day fix, step by step
Day one: test everything, not just chlorine. You need free chlorine, pH, alkalinity, cyanuric acid, and phosphates. High cyanuric acid is the classic trap — it locks up chlorine so thoroughly that shocking does nothing. If CYA is over 80 ppm, a partial drain and refill comes first, or nothing else works.
Day one through two: brush every surface, then shock hard — a real breakpoint dose, not a courtesy scoop. Run the pump 24 hours. The water will turn from green to cloudy gray-blue. That's not failure; that's dead algae.
Day three through four: clean or backwash the filter daily — it's now doing the heavy lifting — and add a clarifier if cloudiness lingers. Keep chlorine elevated; a bloom rebounds fast if levels sag mid-recovery.
Day five: retest the full panel, rebalance pH and alkalinity, and vacuum the settled dead algae to waste, not through the filter. Water should be clear to the bottom. If it isn't, the filter media itself is usually the culprit — sand channels, cartridges clog, DE grids cake.
How to never meet the green pool again
The boring answer is the right one: consistent sanitation, verified weekly with a real test kit rather than test strips read by eye. Test after heat waves, storms, and pool parties — the three events that consume chlorine fastest. Keep cyanuric acid under 50 ppm if you're on tablets, because it only ever accumulates.
And if the pool does turn, treat it in the first 24 hours. A bloom caught on day one is a one-visit fix. A bloom left a week is the five-day plan above — plus filter work.
