Living algae cells contain chlorophyll and other pigments that give pool water its green color. When chlorine oxidizes these cells, the cell walls rupture, releasing the pigments and cellular contents into the water as microscopic white particles. The green color disappears because the living cells are gone — but the released material creates a white or grey cloudiness that persists until the filter removes it.
The cloudiness is a confirmation signal: it means the shock treatment worked. If the pool stayed green after shocking (rather than turning cloudy), that would indicate the algae wasn't killed — a dose or pH problem.
| Time After Shocking | What to Expect | What to Do |
|---|---|---|
| 0–6 hours | Pool may still look green or grey-green | Run pump, do not re-shock yet |
| 6–12 hours | Color shifting from green toward grey/white cloudy | Backwash or clean filter |
| 12–24 hours | White/grey cloudiness, floor may be visible | Backwash again, add clarifier |
| 24–48 hours | Progressively clearer; fine particles settling/filtering | Vacuum settled algae to waste |
| 48–72 hours | Should be clear or nearly clear | Retest and balance chemistry |
Dead algae particles are removed by the filter — but only when water is circulating. Run the pump 24/7 until the pool is clear. Stopping the pump allows particles to settle on the floor instead of being filtered out.
Dead algae clogs the filter rapidly. A clogged filter cannot capture new particles and reduces flow — slowing the clearing process. For sand filters: backwash every 6–8 hours until pressure returns to normal baseline. For cartridge filters: remove and rinse cartridges every 6–8 hours. For DE filters: backwash and add fresh DE.
Pool clarifier causes fine particles to clump together (flocculate) into larger particles that the filter can capture more easily. Add clarifier per label directions 24 hours after shocking. Do not add clarifier at the same time as shock — the high chlorine will degrade some clarifier products.
Once the pool is clear enough to see the floor (usually 24–36 hours after shocking), vacuum the settled dead algae directly to waste — not through the filter. Set the multiport valve to "waste" on a sand or DE filter. This removes the material from the pool without clogging the filter further. Refill water level after vacuuming.
If the pool turns white or grey after shocking and then turns green again within 24–48 hours, the algae was not fully killed. This usually means the shock dose was too low, pH was too high during shocking (reducing chlorine effectiveness), or CYA is too high for the shock to work at normal doses. Test CYA — if above 80 ppm, a partial drain is needed before re-shocking.
Not all white cloudiness is dead algae. Other causes of white cloudiness after shocking include:
True dead algae cloudiness is consistent, improves steadily over 24–72 hours, and is not accompanied by scale formation.
PoolLens logs shock doses and tracks the clearing progression from green to clear. Know when to backwash, when to add clarifier, and when the treatment is complete — all in one place.
Open PoolLens Free →24–48 hours with continuous pump operation and regular backwashing for light algae. Heavily infested pools may take 48–72 hours. Add a clarifier and vacuum settled material to waste to speed the process.
Grey or white cloudiness after shocking is dead algae — the correct treatment progression (green → cloudy → clear). Do not add more chemicals. Keep the pump running, clean the filter regularly, and it will clear progressively over 48 hours.
Dead algae cells aren't toxic, but wait until the water clears and chemistry normalizes (FC 1–3 ppm, pH 7.2–7.8) before swimming. Fine particulate matter can irritate eyes and skin even at correct chlorine levels.
Always vacuum to waste — not through the filter. Dead algae overwhelms filters quickly. Set the multiport valve to "waste" and vacuum slowly. This is faster and protects the filter from clogging. Refill water level after vacuuming.
The algae wasn't fully killed. Likely causes: dose was too low, pH was too high during shocking, or CYA is above 80 ppm making chlorine ineffective. Test CYA — if high, do a partial drain first. Then re-shock at the correct dose for the algae severity.