What is Pool Shock and Why It Matters
Pool shocking, also known as a shock treatment, is adding a concentrated dose of chlorine or other oxidiser to your water. It kills bacteria, breaks down chloramines (combined chlorine), clears up algae, and removes organic contaminants that regular daily chlorine can’t handle on its own.
Think of it as hitting the reset button on your pool water. Whether it’s after a big storm, a weekend party or just part of your regular maintenance routine, shocking helps get your pool water back to safe, clean swimming conditions.
There are different types of shock products out there, from traditional chlorine based to non-chlorine oxidisers. Each works a little differently but the goal is always the same: clearer, cleaner, safer pool water.
When to Shock Your Pool?
Here are the signs and scenarios in which you’ll want to shock your pool water::
- The water is cloudy or green
- There’s a chlorine or musty smell
- After a big pool party or kids’ day
- After a storm or heavy rain
- You see algae or bacteria
- Combined chlorine levels are over 0.5 ppm
- As part of your weekly or biweekly routine in the warmer months
Pro Tip: Even if your water looks good, regular shocking (especially in the summer) prevents problems before they start. Additionally, always shock your pool at night. Sunlight breaks down chlorine fast, so it’s much less effective during the day.
4 Steps to Shock Your Pool
Step 1: Test and Balance Your Water
Test your pool water with a good test kit or strips. pH should be between 7.2 and 7.6. Also check your alkalinity, it should be between 80 and 120 ppm. Take note of your chlorine levels too. If combined chlorine is above 0.5 ppm, that means it’s time to shock. Balancing your water first is key so the shock treatment works as intended.
Step 2: Prepare the Shock Treatment
Depending on the type of shock you’re using, you may need to pre-dissolve it. Granular shock needs to be mixed in a bucket of water before adding to the pool. Stir well and then pour the solution around the pool edge. Some fast-dissolving products can be added directly but always check the instructions before you do.
Step 3: Shock at the Right Time
For best results, shock your pool in the evening or at night. Sunlight can break down chlorine fast so the treatment won’t work as well. Once you add the shock, run your pool filter for 8 to 12 hours to circulate the water and distribute the chemicals. Also brush the walls and floor of the pool during this time to loosen debris and let the shock reach every surface.
Step 4: Test Before Swimming
Don’t rely on the clock alone to know when it’s safe to swim again. The key is to test your chlorine levels and wait until the free chlorine drops to 3 to 5 ppm. Depending on the type and amount of shock used, this can take 12 to 24 hours. Always test the water first before anyone gets back in the pool.
Here’s what not to do when shocking your pool:
❌ Don’t shock in direct sunlight
Sunlight destroys chlorine, especially Cal-Hypo. So your shock won’t work as well.
❌ Don’t skip the pH test
If your pH is off, the shock won’t work. Always check first.
❌ Don’t pour granular shock directly (unless directed)
Some pool liners or surfaces can get bleached or damaged. Pre-dissolve to avoid this.
❌ Don’t use the skimmer
Never add shock through the skimmer, especially if you have an automatic chlorinator. It can cause bad reactions or damage your system.
❌ Don’t swim too soon
Wait until chlorine levels drop to a safe range. Swimming too early can irritate skin and eyes.
❌ Don’t rely on shocking alone
Shocking works best as part of a complete maintenance routine. Brush, skim, vacuum, and run the filter.
Types of Pool Shock (And Which to Use)
Knowing the different types of pool shock helps you choose the right one for your pool. While all are designed to clean and rebalance your water, they work slightly different and are good for different situations. Here’s a breakdown of the most common types and when to use each.
Calcium Hypochlorite (Cal-Hypo)
This is one of the most popular and strongest forms of pool shock. As an unstabilised chlorine product it’s great at killing algae, bacteria and clearing up cloudy or green water — but it’s sensitive to sunlight and should be applied after sunset. Cal-Hypo works fast and is good when your pool needs a hard reset, like after an algae outbreak or heavy storm. Just note it raises calcium levels so it’s not good for pools in hard water areas. Most Cal-Hypo needs to be pre-dissolved in a bucket before adding to the pool.
Use Cal-Hypo when your pool is cloudy, green or has an algae bloom — especially after heavy use or rain.
- Strong, fast acting chlorine
- Best used at night
- Raises calcium levels
- Needs pre-dissolving
Dichlor
Dichlor is a shock chlorine with cyanuric acid (CYA) to prevent chlorine from being destroyed by sunlight. Good for routine maintenance if you need to shock during the day. While convenient, frequent use can build up CYA which can reduce chlorine’s effectiveness over time. Use dichlor sparingly if you already use stabilised chlorine tablets or if your CYA levels are rising.
Use Dichlor for weekly or biweekly maintenance if you need to shock during the day and your stabiliser levels are low.
- Stabilised chlorine with CYA
- Daytime use
- Good for maintenance
- Overuse builds up CYA
Potassium Monopersulfate (Non-Chlorine Shock)
This is a chlorine free oxidiser that breaks down organic material and chloramines without raising chlorine levels. It doesn’t kill algae or bacteria but is great for reducing odours and keeping water clear. Because it’s chlorine free you can swim again in a few hours – often in just a few hours. Perfect for indoor pools, spas or sensitive swimmers. Also great as an in-between treatment if you have a chlorine free or low chlorine system.
Use non-chlorine shock to eliminate odours, reduce swimmer irritants and maintain water clarity, especially in indoor pools or after heavy use.
- Chlorine free oxidiser
- Doesn’t kill algae or bacteria
- Indoor pools and spas
- Swimmable in hours
Recommended Use
- Cal-Hypo when the pool is dirty, cloudy, green or recovering from heavy rain, algae blooms or high bather loads. Best used at night.
- Dichlor for weekly or biweekly maintenance when CYA level is low and you need to shock during the day. Don’t overuse if you already use stabilised chlorine tablets.
- Potassium Monopersulfate when you need to oxidise contaminants without raising chlorine levels — indoor pools, spas or before events where fast swim access is needed.
Clean, Safe
Shocking your pool isn’t just about fixing a problem, it’s about preventing one. Whether you’re cleaning up cloudy water or staying ahead of bacteria, the right shock will keep your pool clear, balanced and safe all season.
Ready to take control of your pool water? With a little prep and the right process a pool shock will bring your backyard paradise back to life, and keep it there.
