Green water is algae, and one big dose of shock will not clear it. Test CYA and pH, brush the pool, run the pump around the clock, then raise free chlorine to SLAM level — 40% of your CYA reading — with liquid chlorine and hold it there until the water clears and chlorine stops dropping overnight. A light bloom takes a couple of days; a deep one, about a week.
Why Your Pool Turned Green
Green water is algae, and algae only blooms when your free chlorine loses the ability to kill it faster than it grows. That almost always means one thing: free chlorine dropped too low for too long. A day or two of a dead pump, a broken feeder, a heatwave burning off chlorine, or a stabilizer level so high that your chlorine went weak — any of these opens the door, and a bloom can turn a clear pool pea-green overnight.
The colour tells you how far it has gone. A faint green tint or slightly cloudy teal is an early bloom you can clear in a day or two. Deep green you can't see through, or dull swampy green with slippery walls, is an established bloom that needs several days of sustained work. Either way the cure is the same idea: raise chlorine high enough, and hold it there long enough, to oxidise every last algae cell.
- A dead pump — a day or two with no circulation is enough.
- A broken feeder that stopped dosing without anyone noticing.
- A heatwave burning chlorine off faster than it is replaced.
- CYA too high, which leaves the chlorine present but too weak to kill anything.
The mistake that keeps pools green
Dumping in one bag of shock, watching chlorine spike for a few hours, then letting it fall. Algae survives that easily. You don't kill a bloom with one big dose — you kill it by holding a high level continuously until the water clears. That is the whole logic of the method below.
First Moves — Before You Add Anything
Five minutes of prep makes the whole job faster. Do these first:
- Test CYA and pH. Your cyanuric acid (stabilizer) level decides how much chlorine you need — there's no single "shock" number. Lower pH to around 7.2 so the chlorine you add works at full strength and your pH test stays readable through the high-chlorine days ahead.
- Brush and skim. Scrub the walls, floor, steps and corners to knock algae loose so the chlorine can reach it. Net out leaves and debris.
- Run the pump 24/7. Circulation spreads the chlorine and the filter pulls out dead algae. Leave it running until the pool is clear.
- Use unstabilized chlorine. Liquid chlorine (sodium hypochlorite) is ideal — it adds no CYA and no calcium. Cal-hypo works but adds calcium; avoid dichlor/trichlor here, since they pile on the very stabilizer that may have caused the problem.
If your CYA is already very high (over about 80–90 ppm), the chlorine needed to clear the bloom gets impractical and expensive. In that case, partially drain and refill to bring CYA down first — see how to lower CYA — then shock at the lower target.
The SLAM Method — Shock Level And Maintain
SLAM is the reliable, no-gimmick way to clear a green pool, popularised by the Trouble Free Pool community. The idea is simple: raise free chlorine to a "shock level" set by your CYA, then hold it there — testing and topping up several times a day — until the pool passes three completion tests. You are not adding chlorine once; you are maintaining a killing level for as long as it takes. The summary below is enough to clear a green pool; the full procedure — prep, top-up doses by volume, and each of the three exit tests in detail — is in how to SLAM a pool.
The higher your CYA, the higher the target — because stabilizer weakens chlorine. Roughly:
| CYA | Shock (SLAM) FC target | Normal FC (after) |
|---|---|---|
| 30 ppm | ~12 ppm | 2–4 ppm |
| 50 ppm | ~20 ppm | 3–6 ppm |
| 80 ppm | ~31 ppm | 4–8 ppm |
| 100 ppm | ~40 ppm | 5–10 ppm |
Add chlorine in the evening so the sun doesn't burn it off before it works, then keep it at target. Test with a good FAS-DPD kit every few hours at first — algae eats chlorine fast, so it will drop quickly early on. Brush daily, run the pump non-stop, and backwash or clean the filter whenever pressure climbs about 25% over its clean reading (with a green pool the filter often needs cleaning every day).
You're done when all three are true
- Clear water — you can see the bottom of the deep end.
- Overnight chlorine loss under 1 ppm — test FC at dusk and again before sunrise; a drop of 1 ppm or less means nothing is left feeding on it.
- Combined chlorine at or below 0.5 ppm.
Hit all three and you can let chlorine drift back down to normal. Miss any one and you keep maintaining shock level.
Get Your Exact Shock Dose
Enter your pool volume, CYA and current chlorine — the calculators turn your SLAM target into the exact amount of chlorine to add each time.
When It's Not Ordinary Green: Mustard & Black Algae
Most green pools are common green algae, which dies to chlorine alone once you reach and hold SLAM level. Two cousins are tougher and need extra steps.
-
Mustard (yellow) algae — chlorine-resistant
A dusty yellow-green film on walls and floor that brushes away in a cloud but comes straight back, often clinging to shady spots. It resists normal chlorine, so it needs a higher, held shock level plus a polyquat-60 algaecide, and it hides on everything that touches the water. Complete a full SLAM first, then raise FC to the elevated mustard level for 24 hours while you scrub the entire pool.
Critically, decontaminate anything that went in the water — poles, brushes, floats, ladders, swimwear, even the vacuum — by soaking or wiping with chlorine. Miss one item and it re-seeds the pool days later.
-
Black algae — rooted and armoured
Dark blue-black spots, usually in plaster/gunite pools, with roots that dig into the surface and a waxy protective cap chlorine can't penetrate. You have to break that cap mechanically: scrub each spot hard with a stiff or stainless brush (a chlorine tablet rubbed directly on the spot helps), then hold SLAM-level chlorine and add algaecide. Expect several rounds over days — black algae rarely dies in one pass.
What Recovery Looks Like
Recovery runs in three stages, and the middle one looks like a setback but isn't:
- 1.Green fades to cloudy grey or milky white. That's the dead algae now waiting for the filter to remove it — a good sign, not a failure.
- 2.Cloudy clears to blue as the filter catches up. Don't switch to a clarifier or panic-shock again; just keep filtering. A light bloom often clears in a couple of days, a deep swamp can take a week of continuous filtering with the filter cleaned daily.
- 3.Done when you can see the bottom and free chlorine has fallen back into the normal range for your CYA. Don't swim before both are true.
If the water has gone cloudy but chlorine is now holding overnight, you've won the algae fight and it's a filtration job from here. Our cloudy pool water guide covers finishing it off, including when a clarifier or flocculant actually helps.
Stop It Coming Back
Green pools are almost always a chlorine-management failure, so prevention is about never letting free chlorine dip too low relative to your CYA.
- Hold the FC/CYA ratio. Keep free chlorine at roughly 7.5% of your CYA as a minimum floor — that's the real target, not a flat "1–3 ppm". Test at least twice a week in swim season.
- Watch CYA. If you rely on trichlor tabs, CYA creeps up and quietly weakens your chlorine until a bloom appears "for no reason". Keep CYA in range and switch to liquid chlorine if it climbs.
- Circulate and brush. Run the pump enough to turn the water over daily, and brush weekly — algae takes hold first in dead spots and shaded corners.
- Phosphates are food, not the fight. A phosphate remover starves future algae, but it's a preventive extra — it will not clear a live bloom. Fix chlorine first, then consider phosphates for maintenance.
If you're still learning how the five numbers fit together, our Pool Chemistry 101 guide explains the chlorine/CYA relationship that decides whether your pool ever goes green again.