Why shock level depends on CYA
Cyanuric acid protects chlorine from sunlight by holding most of it in reserve. The more CYA in the water, the less active chlorine you get at the same reading. So "add a bag of shock" works at CYA 20 and barely does anything at CYA 80. The fix is to scale the target to the CYA: about 40% of CYA for algae and about 60% for mustard algae.
| CYA | Shock FC | Mustard algae FC |
|---|---|---|
| 0 ppm | 10 ppm | 12 ppm |
| 30 ppm | 12 ppm | 18 ppm |
| 40 ppm | 16 ppm | 24 ppm |
| 50 ppm | 20 ppm | 30 ppm |
| 60 ppm | 24 ppm | 36 ppm |
| 70 ppm | 28 ppm | 42 ppm |
| 80 ppm | 32 ppm | 48 ppm |
How to clear a green pool
- Test CYA first. Above about 80 ppm, drain part of the pool before shocking (use the drain tab in the chemical calculator).
- Lower pH to about 7.2–7.4 so the chlorine works harder.
- Raise free chlorine to the shock level. Brush walls and floor, run the pump 24/7, and clean the filter as it loads up.
- Test 2–3 times a day and top up so FC stays at the target. Letting it drop lets the algae recover.
- You're done when the water is clear, combined chlorine is 0.5 ppm or less, and the pool loses 1 ppm or less overnight.
Why not trichlor or dichlor?
They add CYA along with chlorine. Each 10 ppm of chlorine from dichlor adds about 9 ppm of CYA, which raises the shock level you need next time. Use liquid chlorine, or cal-hypo if calcium hardness has room.