Pool shock is a high-dose chlorine treatment used to destroy combined chlorine (chloramines), kill algae, and clear turbid water. Shocking is necessary regularly, not just in emergencies.
Key Facts
- Shock weekly during peak swimming season and immediately after heavy bather load or storm.
- Always shock after dark — UV destroys high chlorine concentrations before they can work.
- Calcium hypochlorite (cal-hypo) is the most common and effective shock type.
- Stabilised shock (dichlor) adds CYA with every dose and should not be used repeatedly.
Why Pools Need Shocking
Chlorine reacts with bather waste, algae, and organic contaminants in the water. Some of these reactions produce combined chlorine (chloramines), which are poor disinfectants and the source of eye irritation and pool odour. Regular free chlorine doses cannot destroy chloramines efficiently — reaching breakpoint concentration is required. Additionally, algae establish faster in pools with even brief periods of low chlorine, and shock levels of chlorine are needed to kill established algae cells.
Types of Shock
Calcium hypochlorite (cal-hypo) is the standard pool shock, sold as a granular powder in 1-lb bags at 65–73% available chlorine. It does not add CYA and is ideal for breakpoint chlorination. Lithium hypochlorite is similar but faster-dissolving and suitable for vinyl liners that could be bleached by undissolved granules. Sodium dichloro-isocyanurate (dichlor) is a stabilised shock — it works quickly but adds CYA with each dose, so it should not be used repeatedly through a season. Non-chlorine shock (potassium monopersulfate or MPS) oxidises organic contaminants but does not raise free chlorine — it is useful after each use of a hot tub.
How to Shock Correctly
Test and record current chemistry before shocking. Calculate the dose needed based on pool volume and target ppm increase using the shock calculator. Add the dose after sunset. For granular shock, pre-dissolve in a bucket of pool water first — never add granules directly to the skimmer or onto a vinyl liner. Broadcast the dissolved solution around the pool perimeter with the pump running. Run the filter overnight and test free chlorine in the morning. Do not allow swimming until FC drops to 5 ppm or below.
Examples
Every Friday evening, a pool owner tests their 20,000-gallon pool (FC 2.5, TC 3.1, so CC 0.6 ppm). The CC is above 0.5 ppm. They calculate a dose to raise FC by 6 ppm (10 x 0.6 = 6): the shock calculator indicates 3.5 lbs of 65% cal-hypo. They pre-dissolve it in a bucket, broadcast after dark, and run the pump overnight. Saturday morning FC reads 3.0 ppm — normal. The pool is clean, clear, and ready for the weekend.
Common Mistakes to Avoid
- Shocking during daylight hours — most of the chlorine is destroyed by UV before it can work.
- Adding granular shock directly to the skimmer, which concentrates it and can damage equipment.
- Using dichlor shock every week and not testing CYA, allowing it to build to levels that make the pool unmanageable.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference