If you want your pool to sparkle all year round, shocking your pool should be part of your monthly maintenance routine. In fact, it’s like a booster shot for your pool! But some pool owners don’t know how to shock their pool and others don’t do it at all (big mistake!), leaving their pool vulnerable to bacteria or algae. Not only does this make your splash zone unsafe, but it can also reduce your pool’s longevity.
So what does it mean to shock your pool? Is it different to manual chlorination? And what kind of pool shock should you use? In this article, we cover everything you need to know about pool shock, like what it is, why it’s important and how to choose the best one for your pool. Then we show you how to shock your pool in 7 easy steps. Ready for some shock treatment? Then let’s dive in!
It would be great if your pool could stay clean all year round, but the reality is that sunlight, rain, heavy use and debris can all affect their chemistry. And while sanitising agents do a good job, they evaporate quickly, leaving your pool vulnerable to pathogens. That’s where pool shock comes in. It boosts your chlorine levels quickly and reduces maintenance down the track. Here’s a breakdown of how it works:
When chlorine mixes with sweat or urine, it forms chloramines, also known as combined chlorine (not to be confused with free chlorine, which actively cleans your pool water). When combined chlorine exceeds a certain level, it stops free chlorine from sanitising your pool and allows pathogens to proliferate. Telltale signs are cloudy pool water, skin or eye irritation and a chlorine smell (commonly found at indoor public pools). If your combined chlorine levels are higher than 0.2 ppm, applying a shock treatment will oxidise the chloramines and rebalance your pool water.
If it rains or your pool is heavily used, chlorine levels can drop dramatically. This can make your pool go cloudy almost overnight. By shocking your pool, you get to increase chlorine levels quickly and stop nasties in their tracks. This is handy if you’ve just had a pool party or you want to use your pool the next day.
Another time to raise sanitiser levels is when you close your pool. Shocking will balance your water chemistry and stop your pool from becoming a breeding ground for bacteria or algae. (Note: A winter algaecide can also help with this.) Shocking when you open your pool in spring is also a good idea. It kills any bacteria or mould spores that may have formed, particularly if you weren’t rigorous with your off-season maintenance.
Most people think that algae is only treated with algaecide, but that isn’t entirely true. The most effective treatment for algae blooms is chlorine – in large doses! That’s because it ruptures the cell walls of algae and stops them from multiplying. Algaecides can be used as a preventative measure after you’ve shocked your pool. That’s because they work in conjunction with chlorine to keep algae at bay, especially if you close your pool in winter.
Before shocking your pool, it’s important to know the different types of pool shock you can use, what’s in them and how they work.
Calcium hypochlorite shock |
Sodium dichloroisocyanuric acid shock |
Sodium hypochlorite |
Non-chlorine pool shock |
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| Calcium hypochlorite, or cal hypo, is a popular and affordable pool shock. Available in granular form, it needs to be dissolved in water before you add it to your pool. Typically, it contains 65–75 per cent chlorine and doesn’t include a stabiliser, so it’s best applied in the evening. One thing to remember is that calcium hypochlorite may increase your calcium levels, so check your water balance regularly to make sure it doesn’t become a problem. | Usually known as dichlor shock, this product comes in granular or liquid form and can be used for regular dosing or shocking. It contains 50–60 per cent chlorine and about 0.9 ppm of cyanuric acid (CYA) for every ppm of free chlorine. This means it stabilises chlorine and sanitises your pool for longer. However, you’ll need to keep an eye on your CYA levels as too much stabiliser can make your chlorine ineffective. If this happens, you’ll need to partially drain your pool to get your levels within the correct range again (i.e. 30–50 ppm). | Sodium hypochlorite is one of the most popular pool chlorines in Australia. Also known as liquid chlorine or household bleach, it has 12 per cent chlorine and no CYA, which means it needs to be added at night. It can be poured straight into the water and effectively sanitises and oxidises pathogens in your pool. On the negative side, you need to use a lot more to achieve high sanitation levels, and it can be corrosive if not used correctly. | This shock is for pool owners who want to avoid using chlorine. Made from potassium peroxymonosulphate (MPS) or sodium percarbonate, non-chlorine shock uses oxygen to remove contaminants and chloramines from the water. This allows free chlorine to do its job more effectively. However, because it’s an oxidiser (and not a disinfectant, like chlorine), it’s not an effective algaecide, so you’ll need to use other products if you want to eliminate algae. On the plus side, it dissolves quickly, it won’t increase Chlorine, CYA or calcium levels, and you can start swimming after 15 minutes. |
Now that you know what pool shock does, it’s time to get down to business! Whether you’re a seasoned pool owner or a newbie, here’s our step-by-step guide to shocking your pool:
Alternatively, drain some of your pool water and top it up to dilute the chlorine. On the other hand, if your chlorine level is too low, shock harder and test again. Once all your levels are within the correct range, you’re ready to start swimming!
There’s a lot of debate in the pool industry about how often you should shock your pool. Some say it should be once a week. Others say once a month. Ideally, you should shock your pool as part of your weekly maintenance routine. This keeps it clean all year round and prevents common pool problems further down the track.
In some cases, you can shock more frequently, like after a downpour, heavy usage or an ‘accident’ in the pool. It’s also a good idea to use shock before opening and closing your pool. This will kill – or at least minimise – the growth of bacteria and algae.
Unless you’re using non-chlorine shock, you should always shock your pool in the evening. Why? Because UV radiation and heat break down chlorine, depleting your chlorine levels by up to 90 per cent in as little as 2 hours. This means bacteria will multiply quickly and you’ll need to keep adding chlorine to keep up, which can be expensive and time-consuming.
What’s more, shocking in the evening allows the sanitiser plenty of time to circulate and do its job properly, and you may even be able to take advantage of off-peak electricity rates. This can save you hundreds on energy bills each year, particularly if you use a variable-speed pump.
You can swim in your pool 8 hours after you’ve added shock, as long as your water chemistry is balanced. If chlorine levels are still high, wait longer. Over-chlorination can cause eye, skin or lung irritation. Non-chlorine shock, on the other hand, isn’t as corrosive, so you should be able to jump in after 15 minutes. But every product is different, so check the manufacturer’s instructions before you jump in.
We don’t recommend adding shock to your skimmer. Concentrated doses of chlorine will damage your basket and pool equipment, leading to costly repairs and replacements. Plus, putting shock in your skimmer is dangerous if you’ve got an automatic chlorinator. When the shock and chlorine combine, gases are produced that may cause your chlorinator or filtration system to explode! Always follow the instructions on your label and keep strong chemicals away from your skimmer.
If you’ve got different types of shock on hand, don’t be tempted to combine them. Each product has specific requirements, and mixing them could result in volatile reactions that could cause injury to you or your pool. If you run out of a particular brand of shock, go to the pool shop and restock.
Chlorine is designed for everyday sanitation, while shock is a large dose of chlorine that quickly removes germs and contaminants. Typically, you would need different products for both applications. Having said that, pool shock like dichlor or liquid chlorine can be used for manual dosing as well. The only difference is the quantity. In other words, you need to use more for shocking and less for dosing.
Salt water chlorinators still produce chlorine, so it’s important to raise those levels after heavy use, rain or algae blooms. Most generators have a ‘super-chlorinate’ button that shocks your pool automatically, eliminating the need for manual shocking. (Note: This function can wear out your salt cell if used too often.) If your chlorinator doesn’t have this setting, follow the steps above for shocking your pool. For best results, use dichlor or non-chlorine shock.
The same advice applies to mineral systems, which also generate chlorine to keep your pool sanitised. However, systems like MagnaPool may not require shocking due to the superior filtration powers of glass media and magnesium. However, if you’ve neglected your mineral pool or someone had an ‘accident’, use calcium hypochlorite shock to get it back on track again.
Shocking your pool is faster and easier than you think. Not only does it conquer chloramines and banish bacteria, but it can also save you hours of pool maintenance later on. In general, you should shock your pool once a week during the swimming season – and more after rain or heavy use. But you’ve got an algae infestation, you’ll need to shock twice or more until your water runs clear again. Here’s a quick rundown of the process:
Want more advice on keeping your pool in tip-top condition? Then check out our Pool & Spa Guides or chat with one of our approved dealers.
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