Pool turned green? Enter your pool size and stabilizer (CYA) reading and get the full battle plan: target level, first dose, and what to do each day until it's clear.
The plan, start to finish
Day 1: Test CYA and chlorine (or accept the defaults above). Add the first dose at dusk, pump on. Brush the whole pool — algae hides in a biofilm that brushing breaks open. Retest in 2–3 hours and dose back up to the shock level. Repeat before bed.Every day after: keep the pump running 24/7, retest morning/afternoon/evening, re-dose to the shock level each time, brush daily, and clean the filter whenever pressure climbs. You're done when the water is clear, you lose less than 1 ppm of chlorine overnight, and the water holds chlorine like it used to.
The math behind the number
shock level ≈ 40% of CYA (minimum 10 ppm if CYA is very low)
dose fl oz = 10.6 × (12.5 ÷ strength %) × (gallons ÷ 10,000) × ppm needed
Stabilizer (CYA) protects chlorine from sunlight but also mutes its killing power, so the level that actually kills algae scales with it. That's why "just dump in a bag of shock" fails at high CYA — the fixed dose that clears one pool barely dents another.
This reach-and-hold approach is often called the SLAM method (Shock Level And Maintain), a name coined by the Trouble Free Pool community — their forum is the deep-dive resource if your pool fights back harder than this page covers.
Why liquid chlorine and not shock powder?
Powdered shocks work but bring passengers: dichlor raises CYA (making your required level climb as you fight), and cal-hypo raises calcium. During a multi-day algae fight those extras accumulate. Liquid chlorine adds nothing but chlorine and a little salt, which is why it's the standard tool for this job.
Frequently asked questions
How much chlorine does it take to fix a green pool?
Enough to reach and HOLD an elevated 'shock level' based on your stabilizer (CYA): roughly 40% of the CYA reading. A 15,000-gallon pool with CYA 40 needs free chlorine held around 16 ppm — often 2 to 4 gallons of liquid chlorine over the first day, then repeated smaller doses until the water clears.
Why does my pool keep turning green even after I shock it?
One big dose kills some algae, then chlorine drops and the survivors regrow. Clearing algae is a siege, not a strike: you hold the elevated chlorine level for days, retesting and re-dosing every few hours at first, until chlorine stops disappearing overnight.
Should I use an algaecide instead?
Chlorine is the workhorse; most green pools clear with chlorine, filtering, and brushing alone. Algaecides are at best a supplement — and copper-based ones can stain surfaces and turn blond hair green. Fix the chlorine first.
How long does it take to clear a green pool?
Light green with good filtration: often 2–4 days. Dark green swamp: a week or more of holding shock level, running the pump 24/7, brushing daily, and cleaning the filter whenever pressure rises. It clears in stages: green → cloudy gray → dull blue → clear.
Can I swim while the pool is green or during shocking?
No — green water means active algae and possibly other microorganisms, and shock-level chlorine is too high for safe swimming. Wait until the water is clear AND chlorine has drifted back down to your normal range.