I opened my pool to a green swamp - how long until it clears?
You pulled the cover off and the water underneath is opaque green, maybe with a floor you cannot see at all. The honest answer up front: a typical green opening clears in four to seven days of continuous work, and a true swamp - black-green, visibility zero, debris on the bottom - can take ten to fourteen. Anyone promising one night is selling something.
The timeline is not random, though. It is decided by three things you control: whether the shock dose was actually sized to your gallons, whether chlorine was held at shock level instead of spiked once, and whether the pump ran around the clock. Get those right and the week goes the way it is supposed to. Here is what that week looks like, day by day, because almost nobody publishes the middle part - and the middle part is where people panic and quit.
Day 1: mechanical work first, then a real dose
Order matters, and it is the same order as a normal opening: get the water circulating, then treat it.
- Net out everything you can see. Leaves and worms on the bottom are chlorine demand. Every handful you remove is chlorine you do not have to buy.
- Get the pump and filter running, and leave them running. Not eight hours. Around the clock, all week. A pump on a timer is the single most common reason a green pool takes two weeks instead of one.
- Test pH and stabilizer before you shock. pH first, because chlorine kills much faster at 7.2 to 7.4 than at 8.0, and opening water often drifts high. Stabilizer (CYA) second, because it decides how big "shock level" actually is - the more stabilizer in the water, the more chlorine it takes to do the same work. If stabilizer sits in the normal 30 to 50 ppm range, shock level means holding free chlorine somewhere around 12 to 20 ppm. If years of trichlor pucks have pushed CYA past 100 ppm, no realistic dose will hold, and part of your fix is a partial drain and refill - no chemical lowers stabilizer.
- Brush the entire pool. Walls, steps, corners, behind the ladder. Algae in a biofilm on plaster shrugs off chlorine that would kill it in open water. Brushing is not cosmetic; it is exposure.
- Dose to your volume, in the evening. This is where most green pools stay green. A gallon of 12.5% liquid chlorinator raises a 10,000 gallon pool by roughly 12 ppm - which means a 20,000 gallon pool needs about two and a half gallons just to reach 15 ppm from zero, before the algae eats any of it. One bag of granular shock in that pool moves the needle a couple of ppm and vanishes by morning. Pour liquid chlorine slowly around the edge with the pump running, and do it at dusk so the sun is not burning it off while it works.
Days 2-3: green turns milky blue, and that is winning
Somewhere in the first 48 hours the water shifts from green to gray-green to a milky, blue-white cloud. This is progress, not a new problem. Green water is live algae. Milky blue water is dead algae - billions of bleached cells in suspension that the filter has not caught yet. The chlorine has done its job; the workload has moved to the filter.
Two mistakes end recoveries right here:
- Letting chlorine sag. Dead algae releases organic material that keeps consuming chlorine for days. Test free chlorine morning and evening - during the worst of it, midday too - and re-dose back to shock level every time it falls. A pool that gets one big dose on day 1 and nothing after often slides back to green by day 4, and the owner concludes shock "didn't work."
- Ignoring the filter. All that dead algae loads the filter fast. Watch the pressure gauge: when it reads well above where it sits clean - for a sand filter, roughly 8 to 10 psi over - backwash, or hose down the cartridge. Expect to do this daily for a couple of days. A loaded filter passes water without cleaning it.
Keep brushing once a day. It feels pointless in opaque water. It is not.
Days 4-6: cloudy blue clears from the top down
The cloud thins, you start to make out the top step, then the shallow-end floor, then the drain. Two other things happen at the same time, and they are your confirmation the chemistry phase is over:
- Chlorine demand collapses. Where the water was eating 4 or 5 ppm a day, it suddenly holds. The overnight test makes this official: dose to shock level at dusk, test after dark, test again before sunrise. If free chlorine dropped less than about 1 ppm overnight - with no sun to blame - nothing living is left to consume it. Stop re-dosing and let it drift back down into its normal band before anyone swims.
- The water polishes. With demand gone, clearing is purely mechanical, and each pass through the filter takes another fraction of the haze out.
About "how to clear a green pool overnight"
You cannot, and it is worth being precise about why. Killing the algae can genuinely happen in a night - a properly sized liquid chlorine dose does most of its lethal work in hours. But dead algae is a physical object, a few microns across, and getting billions of them out of the water is a filtration job measured in turnovers, not minutes. A typical residential system turns the water over in 8 to 12 hours; a milky pool needs several full turnovers.
The one legitimate shortcut is for the cloudy phase, not the green phase: flocculant. Floc clumps the dead algae and drops it to the floor overnight, and the next morning you vacuum the blanket of sediment straight to waste, bypassing the filter. Done right it can compress days 4-6 into one day. Done wrong - vacuumed to filter instead of waste, or used while algae is still alive - it clogs the filter and buys you nothing. Everything else marketed as overnight is a clarifier at best.
The stall tell: chemistry problem or filter problem?
If you are a week in and still stuck, there are exactly two kinds of stuck, and one test separates them.
Still green, or chlorine vanishing every day? Chemistry. Either the dose was never truly sized to your gallons, or stabilizer is high enough that your "shock level" was never actually shock level for this water. Test CYA and redo the math.
Free chlorine holds overnight, but the water stays milky? Not chemistry - stop buying chlorine. The kill is done and the filter is failing at its half of the job: channeled or worn sand (sand does wear out, on the order of five years), a torn cartridge pleat, a bad DE grid, or water quietly bypassing the media. A pressure gauge that never rises over days of filtering a cloudy pool is the classic bypass tell - the filter is not catching anything. Fix the filter and the same water clears in two or three days.
Making next spring boring
A green opening is nearly always inherited from the previous fall and winter - the pool was closed warm, closed unbalanced, or opened late, and the algae had months of quiet to work. That is its own subject: how to stop opening a green pool every spring covers the closing and opening habits that decide it.
For the recovery week itself, the whole game is dose-to-volume and don't-let-it-sag, and that middle part is honest test-kit work - shock-level readings sit above the range a day-to-day chemistry log is built for. Where Basinly carries the load is the bookends. The opening checklist in the Care tab keeps the mechanical steps in order - its last step is literally "Shock and balance the water" - and once chlorine has drifted back down into its normal band, logging picks up again and the treatment plan sizes every routine dose to your pool's actual gallons instead of "a bag." The stability score settling high and staying there is the confirmation you are after: that is when the swamp is officially over.