Waking up to find your swimming pool transformed into a dark green swamp is one of the most frustrating experiences for any homeowner. Whether caused by an unexpected heatwave, neglected sanitization, or a malfunctioning chlorinator, the culprit is almost always chlorophyta—single-celled planktonic green algae.
Many pool owners waste money on retail flocculants, copper algaecides, and clarifiers that cloud the water further. In this guide, we outline the proven SLAM process (Shock Level and Maintain) that clears green pools cleanly and permanently.
Phase 1: Physical Preparation & Organic Debris Extraction
Chemicals cannot sanitize water efficiently if thick decaying leaves and organic silt line the bottom of your pool. Chlorine will spend all its oxidation energy breaking down solid leaves rather than killing microscopic algae cells.
- Skim & Leaf Rake: Use a deep-pocket leaf rake to scoop all solid leaves and twigs from the pool floor.
- Brush the Surfaces: Vigorously brush all walls, floor steps, corners, and behind the pool ladder with a nylon pool brush to disperse the colony into the water column.
- Clean the Filter: Backwash your sand or D.E. filter thoroughly, or rinse your cartridge filter elements.
Phase 2: Chemical Baseline Testing
- Test Cyanuric Acid (CYA): Your CYA level dictates your required shock level. If CYA is between 30 and 50 ppm, your SLAM target is 12 to 20 ppm Free Chlorine. If CYA is over 90 ppm, drain a portion of water first.
- Lower Water pH to 7.2 – 7.4: High pH severely impairs chlorine potency. At pH 8.0, chlorine is only 20% effective; at pH 7.2, it is over 65% effective. Use our pH Balancer Calculator to dose muriatic acid down to 7.2.
Phase 3: Executing the SLAM (Shock Level and Maintain)
- Calculate Shock Dose: Open our Pool Chlorine & Shock Calculator and select your SLAM shock preset based on your CYA level.
- Add Liquid Chlorine After Dusk: Add 10% or 12.5% liquid sodium hypochlorite in the evening so ultraviolet sunlight does not degrade the initial shock dose.
- Run Filter 24/7: Never turn off your circulation pump during SLAM. Continuous filtration is essential to capture dead algae particles.
- Test and Re-Dose 2–3 Times Daily: Test Free Chlorine morning and evening. If FC drops below your shock threshold, immediately add more liquid chlorine to bring it back up.
Phase 4: When Is the Pool Officially Clear? (The 3 Criteria)
Frequently Asked Questions
How long does it take to turn a green pool clear?
With aggressive execution of the SLAM method using liquid chlorine and 24/7 pump filtration, mild green pools clear within 24 to 48 hours. Dark swamp pools with heavy organic debris may require 4 to 7 days of continuous shock level maintenance.
Can I clear a green pool without draining it?
Yes, virtually any green pool can be cleared without draining, provided Cyanuric Acid (CYA) is below 80 ppm. If CYA is over 100 ppm, a partial drain is required first to break chlorine lock.
Should I use algaecide to clear a green pool?
No. Algaecides are primarily preventative agents, not eradicators. Many algaecides contain copper, which stains plaster and hair green, or polymeric compounds that cause excessive foaming. Concentrated liquid chlorine is vastly superior and leaves zero residual byproducts.