How to Clear a Green Pool in 3–5 Days

A green pool is dramatic, but almost always fixable in 3–5 days with the right method. We use the SLAM method — Shock Level And Maintain — the same protocol recommended by Trouble Free Pool (TFP) and most pros.

Before you start: test everything

• Free chlorine: will be near zero
• CYA: critical — determines your shock target
• pH: lower to 7.2 before starting (chlorine works faster in slightly acidic water)
• Filter condition: clean it first — a clogged filter can't help you

The SLAM protocol

1. Raise free chlorine to the SLAM level for your CYA (see table).
2. Test FC every 2–4 hours during the day. Re-dose to maintain SLAM level.
3. Brush the entire pool daily, especially corners and steps.
4. Run the filter 24/7.
5. Vacuum dead algae to waste as it drops.
6. Continue until: CC < 0.5 ppm, overnight FC loss < 1 ppm, water is crystal clear.

SLAM target by CYA

Timeline

• Day 1: Pool turns from green to cloudy gray. You're winning.
• Day 2–3: Cloudy gets lighter. Dead algae on the bottom.
• Day 3–5: Water clears. Pass the three SLAM completion tests.

Don't fall for algaecide

“Quick algaecide” products work on light, early algae — but a fully green pool needs real chlorine levels. Algaecide with a SLAM-level chlorine pool just foams up and wastes money.

It's green but my chlorine reads fine

This is the most common version of the call, and it is almost never a bad test kit. A chlorine number on its own means very little — what matters is that number relative to your stabilizer.

• CYA lock is the usual answer.Chlorine's killing power scales against CYA, so 3 ppm of free chlorine at CYA 30 is a working pool and the same 3 ppm at CYA 100 is effectively unsanitized water. If your stabilizer crept past 80, no realistic shock target exists and the honest fix is a partial drain and refill first. The mechanism is in our cyanuric acid guide.
• Phosphates are feeding it faster than you can kill it. After heavy rain, or on a pool topped up with reclaimed-irrigation overspray, phosphate load can outrun a correct chlorine level. You hold SLAM and it still greens. See phosphate control.
• It might be dead algae, not live algae. Water that went from green to a dull gray-green haze while chlorine held is usually killed algae still in suspension. That is a filtration and vacuum-to-waste problem, not a chemistry one — keep the filter running and stop adding chlorine you do not need.
• Mustard algae gets misdiagnosed as green constantly. If it brushes off easily, sits in shade, and returns in the same spot within a day, you are fighting the wrong organism at the wrong target. Go to mustard algae.
• Green water with genuinely clear water is not algae at all — that is dissolved metal, usually copper or iron, and chlorine will not touch it.

You shocked it twice and it's still green

Shocking twice and getting nowhere almost always means one of six things, and none of them is “needs more shock”:

1. You never reached breakpoint — the dose was sized to the label, not to your CYA.
2. You reached it once and let it fall. A single dose that decays overnight is not a SLAM; the level has to be held.
3. CYA is above 80, so there is no reachable target. Drain and refill first.
4. The filter was never cleaned, so you are killing algae and immediately recirculating it.
5. Algaecide went in instead of chlorine, and now the water foams.
6. It is mustard, not green, and mustard needs a higher hold and every piece of pool gear disinfected or it comes straight back.

How fast can this actually clear?

Three to five days is the honest answer for a fully green pool, and the two things that move it are pump turnover and filter capacity — not how much chlorine you throw at it. A 24-hour turnaround happens, but on a mild bloom with a clean filter and a pump sized to the pool, not on a swamp. Anyone promising an overnight fix on a pool you cannot see the second step of is selling you something.

The limiting factor after the algae is dead is almost always filtration: the water is full of suspended dead cells and the filter has to physically remove them. That is why a pool can pass its chlorine tests and still look terrible on day three. If you would rather hand the whole thing over, our green pool cleanup service covers the recovery end to end.

Prevention: why it happened

Green algae shows up when free chlorine drops below the minimum FC/CYA ratio for too long. Common causes: CYA crept up past 80 ppm, tablet feeder ran out during a vacation, salt cell stopped producing, or heavy storm diluted everything below threshold. Find and fix the root cause or you'll be doing this again in 3 weeks.