The Importance of Shocking a Pool

Have you ever wondered how pool shock works? Or, when to shock a pool? You're not alone! Today, we will discuss the importance of shocking a pool and how to keep your water sparkling all season long.


We'll answer the three biggest questions about shocking and oxidizing a pool:

  • Why should I shock my pool?
  • When should I shock my pool?
  • How do I test the chlorine level in my pool?






Why Shock a Pool?

Chlorine levels can fluctuate depending on the bather load and contaminants in the water. Pool chlorine is made to attack and sanitize bacteria so the pool is safe to swim, but will also bound to ammonias and nitrates in water. This type of bacteria comes from debris, bird droppings, sunscreen lotions, etc. The only way to release the bound (chloramines) is by shocking the pool.

To break up the bound chlorine, you will need to reach breakpoint chlorination. This is done by raising the chlorine up to a very high level for a period of time before dropping it down. Breakpoint chlorination will ensure that you are burning off all the contaminants and turning the combined chlorine back in to an effective sanitizer. We typically recommend using a calcium hypochlorite (cal hypo) shock when super chlorinating a pool. Just keep in mind that this type of pool shock has a small amount of calcium and will cloud the water for up to 12-24 hours in certain situations. Leslie's Power Powder Pro is ideal for helping pool owners reach breakpoint chlorination.

Pro Tip: If you can "smell chlorine," this indicates that the chlorine is combined with ammonias and no longer sanitizing the water. Shocking the water will free up effective chlorine and eliminate this smell.

When to Shock a Pool?

Did you know that 80% of pool owners actually under shock their pools? To be on a regular maintenance schedule of shocking and oxidizing, we recommend alternating between a chlorine shock and non-chlorine shock each week, depending on the tested levels of chlorine. For example, use a chlorine-based shock one week, and chlorine-free shock the next week to oxidize contaminants out of the water. This will help reduce the amount of chlorine needed and increase the overall effectiveness of the chlorine.

So, how do you know when to shock a pool? By testing the water each week during the swimming season, you will see the difference between free chlorine and total chlorine levels. This difference is referred to as combined available chlorine, or chloramine level. Chloramines are one reason why some people may experience skin and eye irritation while swimming.

Here are two different ways to easily oxidize the water when the chloramine level is too high:

  • Breakpoint Chlorination (adding enough chlorine to oxidize off contaminants)
  • Non-Chlorine Oxidizer/Shock

Pro Tip: When your chlorine is in the proper range (between 2-4ppm), we recommend using a non-chlorine shock like Leslie's Fresh and Clear to oxidize and burn off the ammonia. The goal is for your Free Chlorine and Total Chlorine to be the same level at all times.

Test, Don't Guess

Test, test, test. If a pool owner adds chlorine over and over again without testing the water, they may be adding chlorine with no benefit. Pool owners are amazed at how much money they save by adding a chlorine-free oxidizer instead of chlorine shock to the pool when the total chlorine levels are appropriate.

We recommend using a DPD Water Test Kit or Leslie's Test Strips to test for chloramines, telling you when to shock. Remember to store water testing equipment indoors and replace every 6 months. Leslie's retail stores also provide a Free 10-Point AccuBlue® Water Test, so stop on by today!

Pro Tip: Turn your pool light on at night and look for particulate floating in front of the light. Add some Fresh and Clear, let the filter run for a few hours. Turn the light back on and notice that the water is crystal clear! Please note: If you don't see a noticeable difference, you may need to clean or inspect the pool filter.

Contact the Experts

We hope this video answered your questions about shocking and oxidizing a swimming pool. For more expert pool advice, make sure to check out our Pro Tips video series!

Have a question or idea for a future Pro Tips episode? We would love to hear from you! Please submit your comments and feedback to today!

Video Transcription

Hello and welcome back to another episode of Leslie's Pool School Pro Tips. I'm Phil O'Haver and this is Jon Martin.


Together we have over 30 years experience in the swimming pool industry and today we want to spend a little bit more time on talking about,


Shocking. The importance of shocking the pool. So a lot of questions have been coming in through Leslie's blog on, why shock the pool? When to shock the pool? Maybe what types of shock, what is shock, when to shock the pool.

And then what is shocking just in general, right? So, we want to really first understand why we need to shock the pool, right? So we first need have to understand, there's a lot of bather load that can get into the water. So what happens in any swimming pool, is that a chlorine is floating around your chlorine tablets, even some liquid chlorine,

Or salt system.

Something out there, salt system. Any type of chlorine you're putting in the water is wanting to go sanitize and attack bacteria, attack different things in water. Well, a couple things that it can't really attack, it actually bonds to, is ammonias and nitrates in the water. And those come from a variety of sources, between debris, bird dropping, every animal dropping in the pool. I mean, bather lotion, sunscreens, cosmetics, all of that kind of stuff. So what happens, chlorine will actually bond with it. And the only way for you to release that bond is going to be, you guessed it, shocking that pool. So we need to reach what we call a 'break point chlorination' to raise that chlorine level up at a really high level for a period of time and drop it back down to ensure that you're burning off all those contaminants out of the water, burning the contaminants into the atmosphere, releasing it up and turning that combined chlorine back into useful effective chlorine.

It's important to understand that chlorine that is combined with ammonia, is not effective at all. It is not killing things, so, if you smell chlorine, actually odor chlorine is bad. It is combined with ammonia so, I hate to say it but you go to some of the public pools probably in your childhood or even now and you smell that chlorine. Oh, it smells clean! The exact opposite was true of that. It was not a safe environment.

You might want to stay away from that.

Right, which is why consumers should be super chlorinating, should be oxidizing, should be shocking. All three of those things I just said are the same word. Super chlorinate, oxidizing, shocking. All three are the same thing. This is why you need to.

80 percent of customers of local pool owners actually under shock their pool. To be on a regular regimen of shocking your pool or super chlorinating the pool, and if you refer one of our last videos that we shot on really the importance of sanitizing. We talked a little bit more about, the combination of shocks too. So you can get one of the chlorine based shocks on one week, another week you can actually balance that out with a non-chlorine shock, but both of those chlorine and non-chlorine shocks are both oxidizing contaminants out of the water, oxidizing that bad molecule combined with that combined chlorine.

I'll tell you, the secret sauce, cause how do you know when to do that? A test kit.

Test kits.

I caution all customers who shop with us, or anywhere, if you own a pool, you shouldn't just be adding chemicals each week because you're used to it. You should be testing. So if you're testing the water, and you see my chlorine level is, my free chlorine is a 2, but my total chlorine is a 3, that gap between there, that 2 and 3, that's one part per million of combined available chlorine.

Which is a lot.

Or chloramine. Which is the stinky chlorine, the bad chlorine, actually some people are a little more sensitive to chloramine, my daughter's one of them. She can't, if there's chloramines in a pool she gets a rash.

A lot of skin, eye irritation as well. When you can smell that.

So when you see that, you know that you need to oxidize that water. Two different ways you can go about doing it. Breakpoint chlorine, which you talked about. So that's just math. It's pretty easy, we could do it for you, call our stores or whatever. We could tell you how much chlorine to add. Or the much simpler approach, is a non-chlorine oxidizer. If your chlorine level is appropriate, so if your chlorine levels are between that 2-4 range, and it's the appropriate range, then use a non-chlorine oxidizer like our Fresh and Clear. Toss that in there and it will oxidize off those contaminants, it'll burn out all that ammonia, then you'll go back and test and you'll see that your free chlorine and your total chlorine are the same, which is the ideal. You always want those to be the same, but test, test, test. If you add chlorine every week, the exact same dose, over and over again, you may just be dumping chlorine in there and not getting the bang for your buck. You're probably wasting money.

My dad for example, I help maintain his pool, he uses a dichlor and he uses Fresh and Clear. And he uses Fresh and Clear more often than he does the dichlor cause there's not a lot of demand in his pool, there's not a heavy bather load, that's not a, so he's just constantly oxidizing. He's amazed at how much he saves as compared to what he did five year ago which was, he had tablets floating, he'd add shock, he'd poor a little splash of acid without ever testing. He says he spends a third of what he used to spend by simply testing it and understanding what he's adding and why.

So John, to your point with your dad, it's a huge advantage when you talk about a chlorine based shock vs a non-chlorine shock and how they balance one another out. Commercial properties love that chlorine free shock, it's our Fresh and Clear, it's a monopersulfate based powder shock.

And does some clarifying.

And it does a very good job of clarifying. I think you referenced it the last video we sent, but I always recommend for customers, if you try this stuff out, do the light test. At night time, turn that light on, if your light doesn't work, call our service department, and we'll get that fixed for you too, right?

We'll put the number on the bottom. It's right here.

Turn the light on, and you'll see some particulates floating around in front of that light, okay. Turn that filter on, that pump on, get it going. Put the recommended dosage of that Fresh and Clear in there, let that filter for a couple hours. Go back out, turn that light on and in front of that light, that water's going to be just crystal, crystal clear. The nice thing is too, it's almost a shock and swim, right? It's 15 minutes that you got to stay out of the pool, for it to be put in there. It's a great partner, a great compliment to your chlorine based shocks. A lot of times if you're floating tabs out there and then hit it with an occasional chlorine based shock, you may only need to be putting that chlorine based shock in on a very rarely basis.

As John mentioned, supplementing that with that Fresh and Clear that's out there. Yeah, Fresh and Clear is really missing from a lot of backyard customers.

From their pantry.

From the pantry. Absolutely.

[inaudible 00:06:28] Radically under utilized. I don't want to make this a plug about Fresh and Clear, I think, it's important we sort of just circle back around and understand that. While we strongly recommend you have a non chlorine shock in your backyard, obviously Fresh and Clear is our brand so we know it's a superior product but you want to have a non chlorine oxidizer in your backyard, and use it regularly. But test, test, test, test, test so you know how much to add. Don't waste money and more importantly, don't have an unsafe swimming environment. You could have a high chlorine level and you can smell it, and you're like, yes, I've got a ton of chlorine in there and it's still not safe. So it's critically important that you understand the importance of shocking, and that you test before swimming or adding any chemicals.

And then if you don't have one of the test kits at home, that we sell at Leslie's. You can pick them up a lot of different places. If you look at this OTO kit, this OTO kit just tests for the total chlorine in the water, which is the good and bad chlorine combined together.

So it doesn't tell you when to shock?

No it doesn't. So you have your DPD kit here, that actually tests for your good and bad chlorine that's out there. If you don't have one of these kits, you can pick one up from a lot of places carry it, but if you bring it in to Leslie's, we do the full 9 point test. And we're able to go ahead and provide you the solutions on how much shock you may need to reach that breakpoint chlorination if you do have some of that combined chlorine that's in the water.

Our water check system is really sophisticated so it'll do the math for you. There's also calculators online, on our blog there's some calculators there to figure out breakpoint chlorination. You can also use test strips, but the test strips are reasonably accurate but you just need to make sure you keep them indoors, don't store them outside. I recommend you replace them every six months, just because depending on how they're stored or if water got in there but I recommend replacing them every six months but it is a viable option to testing if you don't want to do the drops.

Yep, absolutely.

Hopefully this answers some questions for you. If you have any additional questions please go to our blog, and I know you know how to get there cause you're already there watching this video. But don't hesitate to submit questions. That email address that you see there actually comes to our inbox, so you will get a reply from us and/or you may see your question posted as one of our Pro Tips videos. Thank you for watching.

Top Products

Leslie's Power Powder Plus Calcium Hypochlorite Shock


Leslie's Chlor Brite Sodium Dichlor Shock


Leslie's Fresh 'N Clear Chlorine-Free Shock


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Leslie’s makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and Leslie’s assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. Leslie’s assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.