How to Fix a Green Pool Fast
Updated June 11, 2026
A green pool is an algae bloom: free chlorine fell below what your stabilizer (CYA) level demands, and algae took over. The fix isn’t a magic potion — it’s chlorine, raised to the right shock level for your CYA and held there until the algae is dead. Done properly, most pools go from green to clear in 2–4 days. Here’s the exact process, with real doses instead of guesswork.
Why is my pool green?
Algae spores land in every pool, every day — wind, rain, fill water. They only bloom when nothing kills them, which means one thing: your free chlorine spent time at or near zero relative to your CYA. The usual ways that happens: the chlorinator or feeder ran empty, a storm diluted and contaminated the water, a heat wave plus heavy swimming burned chlorine off faster than it was replaced, or CYA crept so high (from years of trichlor tablets) that a “normal” chlorine reading was effectively doing nothing. That last trap is the sneakiest — our CYA & chlorine guide explains why the same 3 ppm that protects one pool is useless in another.
What you need before you start
- Liquid chlorine (or cal-hypo). Unstabilized, fast, and it adds no CYA. Skip “green-to-clean” miracle products and never shock with trichlor/dichlor tablets or granules.
- A test kit that reads free chlorine and CYA. A drop-based (FAS-DPD) kit is worth it here — strips max out exactly in the range you’ll be working in.
- A pool brush and leaf rake. The mechanical half of the job matters as much as the chemical half.
- A working pump and filter. Chlorine kills the algae; the filter is what actually removes it. It will run nonstop for days — make sure it’s up to it.
How to clear a green pool, step by step
Seven steps. The two that decide success: setting the shock level from your actual CYA (step 2–3) and holding that level instead of dosing once (step 5).
Net out debris and brush everything
Scoop leaves and gunk with a leaf rake — organic debris shelters algae and eats chlorine. Then brush the walls, steps, and corners hard. Brushing breaks the algae’s protective layer loose into open water, where chlorine can actually reach it.
Test CYA and free chlorine
Your stabilizer (CYA) level sets the target: the algae shock level is about 40% of your CYA, with a floor around 12 ppm. If CYA tests above ~90–100 ppm, partially drain and refill first — otherwise the shock level is impractically high. The CYA calculator turns your reading into exact gallons to swap.
Get your exact chlorine dose
No guessing, no “a bag per 10,000 gallons.” Enter your volume, CYA, and current chlorine into the pool shock calculator and it gives the precise amount of liquid chlorine or cal-hypo to hit shock level. Never shock with stabilized trichlor/dichlor — they push CYA even higher with every dose.
Dose at dusk, pump on 24/7
Add the chlorine in the evening so sunlight doesn’t burn it off before it can work, and run the pump around the clock from now until the water is clear. Circulation carries chlorine to the algae and dead algae to the filter.
Re-test and re-dose 2–3 times a day
This is the step everyone skips — and why “I shocked it and it’s still green.” Live algae consumes chlorine, so the level crashes within hours. Test morning, afternoon, and evening, and re-dose back up to shock level every time. Hold it there until the green is gone.
Brush daily and keep the filter breathing
Brush the whole pool once a day to re-expose anything settling. Dead algae clogs filters fast: backwash sand/DE or rinse cartridges whenever the pressure gauge reads ~8–10 psi over its clean starting point. A choked filter is the most common reason a “killed” pool stays cloudy.
Confirm the kill, then let it drift down
You’re done when three things are true: the water is clear, free chlorine holds overnight (loses ≤1 ppm with the pump running and no sun), and combined chlorine is ≤0.5 ppm. Then stop dosing and let chlorine fall naturally — since the shock level was set at ~40% of your CYA, you’re already at the safe-swim ceiling, so it’s swimmable as soon as it dips below that (here’s exactly when it’s safe).
Why one big shock doesn’t work
Live algae is a chlorine-consuming machine. Dump in a triple dose tonight and by tomorrow afternoon it can read near zero — the algae you didn’t kill keeps eating, recovers, and you’re back where you started, minus a bucket of shock. That’s the whole idea behind the SLAM method (shock, level, and maintain): the dose matters less than the holding.
How long does it take to clear a green pool?
Honest ranges, assuming you hold the level and filter around the clock:
| Starting point | What it looks like | Time to clear |
|---|---|---|
| Teal / light green | Water tinted but you can still see the bottom | 1–2 days |
| Green | Floor hazy or invisible in the deep end | 2–4 days |
| Swamp green-black | Can’t see the first step; debris on the bottom | 4–7+ days |
Times assume you hold the shock level (re-testing and re-dosing 2–3× a day) with the filter running 24/7. The kill is usually fast — most of the wait is the filter removing dead algae, so a struggling or undersized filter stretches every estimate.
Pool green but chlorine is high?
The most-asked variant of this problem — a green pool that tests fine. Three causes, in order of likelihood:
1. Your test is maxed out, not your chlorine
Test strips and basic DPD kits bleach out at high chlorine and can read zero or normal when free chlorine is actually sky-high — or vice versa. If a reading doesn’t match what you’ve added, dilute the sample 1:1 with distilled water, re-test, and double the result (or use a FAS-DPD kit, which handles shock-level readings properly).
2. The chlorine is there, but CYA has it handcuffed
5 ppm of free chlorine sounds healthy — but with CYA at 100, almost none of it is active. Stabilizer binds chlorine, and only the unbound fraction kills algae. This is why the shock level scales with CYA, and why a pool can be green at chlorine readings that look “high.” If your CYA is over ~90–100, lower it with a partial drain and refill before fighting the algae.
3. Clear green water? That’s metals, not algae
If the water is green but transparent — you can see the bottom through green-tinted water, often right after shocking — the culprit is usually dissolved copper (from well water, copper-based algaecide, or a corroded heat exchanger) oxidizing, not algae. More chlorine makes it worse, not better. The fix is a metal sequestrant and finding the copper source. Algae-green is cloudy or murky; copper-green is clear.
How to keep your pool from turning green again
Algae prevention is one sentence: keep free chlorine matched to your CYA, all the time. In practice that means testing a couple of times a week in summer, knowing your CYA (re-test it monthly — every trichlor tablet raises it), and dosing with the chlorine calculator instead of habit. Make sure the pump runs long enough for a full daily turnover, and brush the spots circulation misses — steps, corners, behind ladders — weekly. A pool that holds 3–4 ppm against a CYA of 40 simply doesn’t turn green. And if chlorine keeps vanishing even though the water looks clear, diagnose it before the bloom shows up — see why your pool won’t hold chlorine.
Common questions
How long does it take to clear a green pool?
Most green pools clear in 2–4 days if you hold free chlorine at shock level (re-dosing 2–3 times a day) with the filter running 24/7. A light teal tint can clear in 1–2 days; a black-green swamp can take a week or more. Anyone promising a dark green pool fixed in 24 hours with one product is selling something — the chlorine kill is fast, but filtering out the dead algae takes days.
Can you swim in a green pool?
No. Green water means the sanitizer failed, so along with algae the pool can carry bacteria you can’t see — and cloudy green water is a drowning hazard because a swimmer below the surface is invisible. Wait until the water is clear and free chlorine is back at or below the safe level for your CYA — about 40% of your stabilizer reading, or 5 ppm in an unstabilized pool.
Why is my pool still green after shocking it?
Almost always one of three things: you dosed once and stopped (algae eats through a single shock in hours — you have to re-dose to hold the level), the dose was set without knowing your CYA (high CYA demands a much higher shock level), or you shocked with stabilized dichlor/trichlor, which raises CYA further and makes each round less effective. Test CYA, set the right level, and hold it.
Do I need algaecide to fix a green pool?
No — chlorine at shock level is what kills an active bloom; algaecide is a supplement at best. If you want one as a preventative afterward, use Polyquat 60. Avoid copper-based algaecides: copper stains surfaces, turns blond hair green, and can tint the water itself green.
Should I just drain a green pool and start over?
Almost never — even swamp-green pools clear with the shock-and-hold process, and fresh water still has to be balanced and chlorinated anyway. The exception is high CYA: above roughly 90–100 ppm, partially drain and refill first so the shock level is reachable. Never fully drain an in-ground pool; an empty shell can be cracked or floated by groundwater.
Why did my pool turn green overnight?
Free chlorine hit zero — an empty chlorinator or skipped doses, heavy rain diluting the water, a big bather load, or high CYA quietly disabling the chlorine you did have. In 80°F+ summer water an algae bloom can go from invisible to visibly green in a day. The fix for “never again” is keeping free chlorine matched to your CYA level.
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