| What You See | After How Long | Diagnosis | Action |
|---|---|---|---|
| White/grey cloud, improving | 12–24 hrs after shock | Dead algae — SUCCESS | Filter continuously, backwash every 6–8 hrs |
| Still green, same shade | 24 hrs after shock | Shock failed — check pH/CYA/dose | Fix root cause, re-shock |
| Green got lighter then stayed | 24–48 hrs after shock | Insufficient dose — not full kill | Re-shock at higher dose |
| White cloud, not improving | 12–48 hrs after shock | Calcium precipitation (hard water) | Lower pH with muriatic acid, filter |
| Went from green to green (darker) | 24 hrs after shock | Algae multiplying — shock made it worse? | Test CYA — if above 80 ppm, drain first |
The green-to-grey-to-clear progression is what you want. It means:
The correct response: keep the pump running 24/7, backwash the filter every 6–8 hours, and add a clarifier 24 hours after shocking. Do not add more chemicals. Do not panic. The pool will clear within 48–72 hours.
If the pool shows no visible improvement after 24 hours of continuous filtration, the shock did not kill the algae. The three most common reasons:
Chlorine effectiveness drops dramatically with rising pH. At pH 7.2, chlorine is approximately 65% active. At pH 8.0, only about 21% is active. If you shocked without adjusting pH, a high-pH pool may have wasted most of the shock dose.
Fix: lower pH to 7.2 with muriatic acid, wait 4 hours, then re-shock at the full dose for your algae severity.
If CYA is above 80 ppm, chlorine is bound so tightly to the stabilizer that it cannot effectively kill algae — even at high doses. The pool will read free chlorine on a test but the actual available sanitizing power is a fraction of the reading.
Fix: do a 50% partial drain, refill with fresh water, test CYA again (should now be half of original), balance pH to 7.2, then shock at the correct dose.
Green algae requires 2–3 lbs of calcium hypochlorite (65%) per 10,000 gallons — not 1 lb, and not a single trichlor tablet. If the dose was insufficient, algae was stunned but not killed and continues multiplying.
Fix: re-shock at the correct dose. For a pool that has had failed shock, use the moderate-to-heavy algae dose (3 lbs per 10,000 gallons).
After two failed shock treatments, test CYA before doing anything else. High CYA is the single most common reason shock consistently fails. No amount of additional chlorine helps when CYA is above 100 ppm — dilution is the only fix, and shock won't work until you drain and dilute.
Calcium hypochlorite shock raises pH as it dissolves. In pools with calcium hardness above 300 ppm and pH above 7.8 post-shock, calcium carbonate precipitates out of solution as fine white particles. This can persist without improving because the calcium remains suspended.
Fix: test pH — if above 7.8, add muriatic acid to bring it to 7.2–7.4. As pH drops, the calcium carbonate re-dissolves and the cloudiness clears. Run the filter continuously and backwash every 6–8 hours. Add clarifier to speed particle removal.
For any post-shock cloudiness that is clearly the dead algae scenario (improving progressively):
PoolLens logs shock dose, pre-shock pH and CYA, and post-shock progress — so you can identify exactly which variable caused a shock to fail and avoid repeating the same mistake next time.
Open PoolLens Free →Light algae: 24–48 hours. Moderate algae: 48–72 hours. Severe: 3–5 days with multiple shock treatments. No improvement after 24 hours of continuous filtration = shock is not working — check pH, CYA, and dose before re-shocking.
Check three things: pH (above 7.8 = reduce before re-shocking), CYA (above 80 ppm = drain 50% first), and dose (green pools need 2–3 lbs cal-hypo per 10K gallons). Fix the root cause, then re-shock at the correct dose.
White cloudiness post-shock is either dead algae (correct outcome — improves progressively) or calcium precipitation from cal-hypo raising pH in hard water (test pH — if above 7.8, lower with muriatic acid and the cloudiness will clear). Both are fixable with continued filtration.
Yes — 24 hours after shocking (not simultaneously — high chlorine degrades some clarifiers). Clarifier causes fine particles to clump for easier filtration. Speeds clearing by 30–50%. Follow with continuous filtration and backwashing every 6–8 hours.
Almost never for standard algae — but extreme cases (black water, visible slime on surfaces, CYA above 150 ppm) may require a full drain and acid wash before shock can be effective. If CYA is chronically extreme and a partial drain doesn't fix it, a full drain with surface cleaning may be more cost-effective than repeated shock treatments.