Shocking a pool — also called superchlorination — means raising free chlorine to a level that achieves breakpoint chlorination. At this point, free chlorine is high enough (roughly 10x the combined chlorine level) to oxidize and destroy chloramines rather than just add to them.
At normal maintenance levels (1–3 ppm free chlorine), your pool can sustain sanitation but cannot destroy established chloramines or actively attack algae colonies. At shock levels (7.5–10 ppm), it can.
| Situation | Shock Dose |
|---|---|
| Routine weekly maintenance | 1 lb per 10,000 gal (reach 5–7 ppm) |
| Pool smells of chlorine (chloramines) | 1–2 lbs per 10,000 gal (reach 10 ppm) |
| Light green water / early algae | 2 lbs per 10,000 gal |
| Green pool (visible algae bloom) | 2–3 lbs per 10,000 gal |
| Dark green or black pool | 3–4 lbs per 10,000 gal |
| After a fecal incident | Raise to 20 ppm, hold 30+ minutes |
| After a large pool party | 1–2 lbs per 10,000 gal |
| After heavy rainstorm | 1 lb per 10,000 gal |
65–73% active chlorine. Fast-acting. Available in 1-lb bags. Best for algae treatment and routine shocking. Raises calcium hardness slightly over time. Dissolve in a bucket of water before adding.
About 56% active chlorine, includes CYA stabilizer. Works fast and does not require pre-dissolving. Good for saltwater pools or occasional use, but adds CYA — avoid if CYA is already high.
Not a chlorine product — an oxidizer. Oxidizes organic waste and chloramines without raising free chlorine. Safe to swim in 15 minutes. Good for routine weekly maintenance when algae is not a concern. Does NOT kill algae.
Always shock at dusk or night. UV sunlight degrades free chlorine rapidly — daytime shocking wastes up to 50% of the product before it can do its job. Shock in the evening and the chlorine works all night while you sleep.
PoolLens tracks your combined chlorine and pool history. It tells you when your pool is overdue for shock treatment — before you can see or smell the problem.
Open PoolLens Free →Shock when combined chlorine exceeds 0.4 ppm, when you smell chlorine, when you see algae starting, after heavy rain or a pool party, after any fecal incident, or as a weekly preventive during peak summer use.
Calcium hypochlorite (Cal-Hypo) is the most effective for killing algae and eliminating chloramines. Potassium monopersulfate is good for routine oxidation without raising chlorine. Dichlor works fast but adds CYA over time.
Shock begins working immediately. For chloramine removal, the pool is typically clear in 8–12 hours. For algae treatment, allow 24–72 hours before the pool is fully clear depending on severity.
Shock at dusk or night. UV sunlight destroys free chlorine — daytime shocking loses up to 50% to UV burn-off. Night shocking gives chlorine maximum contact time with contaminants.
Overdosing raises free chlorine to unsafe swim levels (above 5 ppm). It can also bleach pool liners, irritate equipment seals, and — if CYA is very low — degrade UV-exposed pool surfaces. Stick to the recommended dose unless treating a severe algae bloom.