What "shock" really means
Shocking is not one big dose. To clear algae, cloudy water or combined chlorine you raise free chlorine to the shock level and hold it there, testing and topping up several times a day, until three checks pass:
- combined chlorine is 0.5 ppm or less,
- the overnight chlorine loss is 1 ppm or less (test at dusk and again before sunrise),
- the water is clear.
Stopping early, the most common mistake, lets algae grow back within days.
Shock levels by CYA
| CYA | Shock level | 10% liquid chlorine from 2 ppm, 15,000 gal |
|---|---|---|
| 20 ppm | 10 ppm | 1.2 gal |
| 30 ppm | 12 ppm | 1.5 gal |
| 40 ppm | 16 ppm | 2.1 gal |
| 50 ppm | 20 ppm | 2.7 gal |
| 60 ppm | 24 ppm | 3.3 gal |
| 80 ppm | 32 ppm | 4.5 gal |
Which shock to use
Liquid chlorine is the cleanest choice because it adds nothing but chlorine. Cal-hypo adds 0.7 ppm of calcium per ppm of chlorine, so a single shock at CYA 40 adds about 10 ppm calcium. Dichlor adds 0.9 ppm of stabilizer per ppm of chlorine, which raises the shock level you need next time. Non-chlorine shock (MPS) oxidizes but does not kill algae.