“Shocking” a pool means raising free chlorine high enough — fast enough — to break down combined chlorine, kill algae, and oxidize accumulated contaminants. It's not a routine weekly task; it's a treatment for a specific problem.
When to shock
- Combined chlorine above 0.5 ppm
- Visible algae (any color)
- After heavy bather load (pool party, thunderstorm of kids)
- After a fecal incident
- After reopening following a neglected period
Breakpoint chlorination: the math
You need free chlorine at roughly 10x the combined chlorinelevel, all at once, to “break through” and burn off chloramines. If CC is 1 ppm, you need FC to reach 10+ ppm.
For algae removal, use the SLAM (Shock Level And Maintain) method: raise FC to the shock level for your CYA, and hold it there until three conditions are met:
- CC is below 0.5 ppm
- Overnight chlorine loss is under 1 ppm
- Water is crystal clear
Shock target by CYA level
| CYA | Shock FC target |
|---|---|
| 0 ppm (indoor) | 10 ppm |
| 30 ppm | 12 ppm |
| 40 ppm | 16 ppm |
| 50 ppm | 20 ppm |
| 60 ppm | 24 ppm |
Shock at dusk, not noon
Sunlight destroys chlorine. Shocking at 2 PM means half your dose is gone before it does any work. Shock at dusk so it has all night to work before UV hits it.
What to use
- Liquid chlorine (sodium hypochlorite, 12.5%): Best for most situations. No CYA added.
- Cal-hypo (calcium hypochlorite, 65–73%): Adds calcium hardness (avoid if CH already high).
- Dichlor: Adds CYA — avoid for routine shocking unless CYA is low.
- Non-chlorine shock (potassium monopersulfate): Oxidizes but doesn't sanitize. Useful supplement, not a replacement.
Half-shocking is worse than not shocking. If you're not going to reach breakpoint, you're just adding chlorine that will combine further and make things worse.