How Do You Fix a Green Pool?

Algae explodes when free chlorine drops below 1 ppm in water above 85°F. That can happen in 48 hours if a pump fails, a salt cell dies, or chemicals weren't refilled. By the time you see green, the algae is already attached to walls and lining — chlorine alone won't dislodge it.

Most green pools are caused by equipment failure, not neglect. The pump stops circulating, water stagnates, chlorine burns off in the sun, and the pool goes green in 3–5 days. After-storm cases are different — debris and runoff overwhelm chlorine demand all at once.

Day 1 is when most of the work happens. Test water chemistry first (especially phosphates), then dose shock chlorine based on the actual gallon count, add an appropriate algaecide for the algae color (green, yellow, or black), and clean the filter cartridge or backwash the DE filter to remove dead algae fragments. The pump runs 24/7 starting now.

Days 2–3 are monitoring days. Test chlorine and pH daily, add more shock if free chlorine drops, and continue brushing walls daily. Water typically transitions from green to cloudy gray-white as algae dies. Days 4–5: the cloudy water filters out, equipment gets a final check, and chemistry returns to normal target ranges.

Target free chlorine: 1–3 ppm normally, but 10–15 ppm during initial shock (10× the algae demand). Target pH: 7.4–7.6. Target phosphates: under 100 ppb — phosphates feed algae, and high phosphates make restoration take longer.

Cyanuric acid stabilizer should be 30–50 ppm BEFORE you shock — if stabilizer is over 70 ppm, chlorine becomes ineffective against algae. This is the number most DIY restorations miss.

Overshocking. Dumping a full bag of granular chlorine into a 20,000-gallon pool sounds aggressive in a good way, but it actually damages plaster, kills salt cells faster, and can stain liner colors. Dose by the gallon count, not by feel.

Skipping the phosphate test. Algae feeds on phosphates from leaves, lawn runoff, and tap water. A phosphate remover (Sequa-Sol or similar) added on Day 1 cuts the algae's food supply and shortens the cleanup by 1–2 days.