Pool problem
Green pool water? Kill algae in 24–48 hours
By Dominik Graham Pritchett · Updated · 7 min read
Green pool water almost always means algae bloom — triggered by low chlorine, high pH, or a heat wave. This guide walks you through the proven 5-step treatment that gets your water clear again in 24 to 48 hours.
Why does a pool turn green?
A green pool is the visible result of an algae bloom. Algae are microscopic aquatic plants that always exist as spores in ambient air and enter the pool via wind, rain, or bathers. As long as free chlorine stays above 1 mg/l, spores are killed on contact. When chlorine drops — after heavy rain, hot weather, intense sun, or a busy pool day — algae multiply explosively within hours and turn the water green. The treatment is always the same sequence: brush and vacuum, correct pH, shock chlorinate, and run the filter for at least 24 hours. Only if water remains green after this treatment are deeper causes at play — like a failing filter or heavy phosphate load from runoff.
All values and recommendations are guidance for private residential pools based on generally accepted industry standards. Commercial installations (hotels, clubs, public pools) are subject to stricter regulations such as Germany's DIN 19643.
Identify the algae
Three algae types — three difficulty levels
Green algae
Most common (95%). Water turns light to moss green, often with slippery film on walls. Treatable with normal shock chlorination per the 5-step plan.
Yellow algae (mustard algae)
Rare but stubborn. Yellow-brown film on shaded walls, few suspended algae. Needs double shock dose and multi-day treatment.
Black algae
Very rare, very resistant. Dark green to black spots deep in grout and tile. Requires manual scrubbing plus specialty algaecide — often weeks.
Diagnosis
The 5 most common triggers
| Cause | How to recognize it | Frequency |
|---|---|---|
| Chlorine too low (< 1 mg/l) | Classic after heat wave or pool party. Chlorine consumed faster than re-dosed. | Very common |
| pH too high (> 7.6) | Chlorine is present but only 20% effective — algae grow anyway. | Very common |
| Heat wave (water > 28 °C / 82 °F) | Chlorine breaks down faster in heat while algae grow explosively. | Common |
| Filter running too little | Under 8 hours per day, or filter sand old (> 3 years). | Common |
| Phosphate input | After heavy rain, blossoms, or fertilizer runoff — nutrient for algae. | Rare |
Immediate fix
The 5-step plan
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Deep brush and vacuum
Brush walls, floor and waterline thoroughly — even areas that look clean. Vacuum settled algae; with sand filters use the waste setting, otherwise algae end up in the filter and get pushed back out.
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Lower pH to 7.0–7.4
Chlorine works strongest in this range. Above 7.4, correct with pH minus — otherwise most of the shock chlorination is wasted. Dose per product label or the app calculator.
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Shock chlorinate
Add 20–40 g chlorine granulate per cubic meter in the evening with pool lights off — noticeably higher than for basic cloudiness. Add algaecide per label for stubborn blooms.
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Run filter 24 hours non-stop
Circulation without pause. Backwash sand filters multiple times — dead algae clog filters fast, raising pressure and dropping flow.
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Recheck after 24 hours
If water is clear and free chlorine is below 3 mg/l, the pool is safe. If green persists, repeat step 3 and inspect the filter.
⚠️ After shock chlorination: free chlorine can briefly hit 5–10 mg/l. Only swim again once it drops below 3 mg/l — otherwise it irritates skin and eyes. In small pools this can take overnight.
Prevention
Keep algae from coming back
- Keep free chlorine at 1–3 mg/l consistently — the only reliable algae defense.
- Test daily in heat waves — chlorine breaks down much faster above 28 °C (82 °F).
- Re-dose immediately after rain — rainwater dilutes chlorine and drops in phosphates.
- Run filter at least 8 hours/day, 10–12 hours in hot weather.
- Cover the pool when not in use — keeps out leaves, pollen and heat.
- Keep chlorine stabilizer (cyanuric acid) at 20–40 mg/l — protects chlorine from UV.
- Use a phosphate remover after any algae bloom — algae need phosphate as fertilizer. Phosphate binders (typically lanthanum chloride or aluminum sulfate) precipitate it out of the water so it gets caught in the next filter cycle. Especially useful after rain periods or leaf debris.
💡 Prevention beats remediation: A single dose of phosphate remover after spring opening (per label, typically 100–200 ml per 10 m³) noticeably reduces algae-vulnerability for the whole season – provided chlorine and pH stay in range.
Pool Buddy walks you through the treatment
The diagnosis flow asks about symptoms and calculates the exact shock dose for your pool volume. Includes a reminder for when to recheck.
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