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Know when to Shock your Swimming Pool

Updated on April 6, 2017
Barack James profile image

Barack is a chemical engineer and pool water chemistry nerd. He has been in the commercial pool maintenance industry for more than 7 years!

Shocking a Non Saltwater Swimming Pool
Shocking a Non Saltwater Swimming Pool | Source

When, Why, and How to Shock a Swimming Pool

Shocking a swimming pool, also known as super chlorination, is a very important process in pool management that every pool owner should understand why they need to shock their pool, how to do it when to do it, chemicals to use, and the best time of the day it should be performed.

Before I changed my nonsaltwater pool to saltwater pool 2 years ago; I used to super chlorinate my pool regularly depending on how immediate the pool needed shocking. Very important to know, there is no single factor that determines how frequent you need to shock a nonsaltwater pool.

Basically, unless there is algae outbreak or pool contaminants such as oil and soil, a saltwater pool does not need shocking much like the non-saltwater pool needs: This is because saltwater pools use chlorine generators to produce chlorine compound similar to the regular chlorine added into the pool during shocking.

Chlorine generators can be adjusted to increase the amount of chlorine in the pool; however, this is not shocking and may be done only prior and/or after heavy pool usage to maintain chlorine at the recommended levels.

Literally, shocking a pool is adding a specific amount of free chlorine into the swimming pool when free chlorine reads below the recommended level of 3 ppm: the first handThis is necessary to kill bacteria, contaminants, and other organisms in the pool water.

Several theories have been put across about the right time or after what duration to shock a swimming pool: However, like I have noted before, there is no single factor that determines pool shocking: Rather, shocking a non-saltwater pool depends on a number of factors, and the best time to shock your pool depends on which factor comes first before the rest.

Depending on these factors discussed below, the amounts of free chlorine reduces drastically to form combined chlorine or chloramine (should not exceed 0.5 ppm); which is ineffective chlorine and is not able to kill bacteria, contaminants, and other organisms such as algae thriving in the pool water.

Personally, I will not recommend a specific period to shock a non-saltwater swimming pool: Rather, I recommend shocking swimming pool depending on how immediate the pool needs shocking: So, the question is not after how long you need to shock your pool: But, when and why you need to shock your pool.

This article includes a get first-hand information from my experience of more than 5 years with nonsaltwater pool; on when, how, and why to shock a nonsaltwater swimming pool, best chemical to use, and the best time of the day to perform it. Enjoy!

1. Shock when Pool Water Temperature Rises

Bacteria and other dangerous organisms such as algae and more thrive in warm water: On the other hand, chlorine effectivity is affected when the pool water temperature rises above the recommend level.

As such, it is appropriate to keep suitable pool water temperature at recommended levels of 80-84° F. You can use a pool thermometer to measure pool water temperature; if the temperature reads higher than the required levels, it is time to shock your pool.

2. Shock when Free Chlorine level goes down

The quantity of free chlorine in the swimming water should be 3 ppm; which should also be very close to total chlorine.Combined chlorine should always be maintained below 0.5 ppm, of 0.0 ppm if possible.

As the free chlorine dissolves in the pool water, chloramine (combined chlorine) forms; which is ineffective chlorine and cannot kill harmful bacteria and organisms, and cannot sanitize water to remove inorganic matters in the pool water.

Most test strips available in the market only measure free chlorine, while you also need to know values of total and/or combined chlorine (free and combined chlorine) before shocking your pool.

I use Lamotte ColorQ Pro 7 digital pool water test kit to find total chlorine in pool water before shocking: I like this equipment since it's accurate, easy to use, and its ability to measure more other pool chemicals and stabilizer such as pH, Bromine, Cyanuric acid, and Calcium hardness.

Important to note; wrong composition of pool chemicals in the water, especially pH and Cyanuric acid affects the effectiveness of chlorine.

As such, you need the best equipment that will give you exact readings of chemicals in the pool water, rather than guessing using test strips.

If you find that chlorine level is much low than 3 ppm, it is time to shock your pool to raise chlorine levels as per the recommendation.

3. Shock after Large Amounts of Rains

Shocking swimming pool after it rains may not be necessary as such: However, to be on the safer side, it is advisable to shock your pool after it rains so heavily.

Heavy rains may carry contaminants into the pool water, which may cause big pool problems if not taken care of in time.

Ideally, the most affected pool chemical by heavy rains is the pH levels. However, when the rain is acidic and alkalinity levels are within the required range, pH concentration will be protected and all you will have to do is to adjust alkalinity levels after heavy acid rains.

Most important; total alkalinity (TA) is very destructive when it get out the balance. Ensure that you closely watch the level of total alkalinity so that it does not exceed the recommended levels of 80 ppm to 120 ppm. Here is how to lower total alkalinity. To raise TA, you can use baking soda.

Moreover, when it rains heavily, particles and leaves may be carried into the pool: As such, it is swimming pool hygiene to clean your pool using large leaf net to remove these particles and leaves to avoid contaminating pool water.

Finally, before shocking, cleaning, or adding any chemical to your pool after heavy rains; ensure that your pool water level is reduced to the normal quantity as long heavy rains may increase pool water levels.

4. Shock During an Extended Period of Hot Sunny Weather

During hot sunny weather, the pool water temperature always rises above the required temperatures for swimming pools.

This affects the effectiveness of chlorine, and you will find that this is the period when pool bacteria and other organisms such as algae find their way into your swimming pool.

Make it a good routine to shock your pool regularly during hot and sunny weather to prevent these harmful bacteria and algae from thriving in your swimming pool.

You can use chlorine stabilizer such as Cyanuric acid to prevent direct UV light from direct sunlight from consuming your chlorine at a higher rate.

5. Shock the Pool when Heavily or Frequently used

Chlorine levels in the pool reduce so drastically when many active swimmers use a pool frequently. As such, you need to measure total amounts of chlorine after heavy swimming, especially in commercial or public pools, and then shock the pool as required.

To be sure with the amount of free chlorine or any other chemical to add to a pool, I use poolcalculator (by Trouble Free Pool),which allows you to enter the reading you get from your favorite pool water test equipment, and bam, you get the correct value of chlorine to add to your pool.

For a saltwater pool, controlling the chlorine levels is easier: All you need to do is raise the saltwater chlorine generator (SWCG) to boost free chlorine prior and/or after heavy usage of the pool.

What is the Best Time to Shock a Swimming Pool?

Ultraviolet (UV) light from direct sun greatly affects chlorine effectivity in the swimming pool. As such, the best time of the day to shock a swimming pool is in the evening when the sun has gone down, to protect free chlorine from UV light from the direct sun.

Which Chlorine Pool Shock is the Best for my Non Saltwater Pool?

Basically, there are 4 types of chlorine pool shocks available in the market: Each pool shock comes with its own pros and cons. The type of pool shock to use in your pool will depend on your pool requirements and some other factors.

Below is more detailed information on different pool shocks available, you can use this information to decide which pool shock chemical is the best for your pool and why it is the best.

1. Calcium Hypochlorite Pool Shock

Calcium hypochlorite is the cheapest type of chlorine shock available in the market.

This pool shock has the highest chlorine percentage (65%) amongst all chlorine pool shocks.

Use calcium hypochlorite if you need to add calcium to the pool water. It needs to be pre-dissolved before it is used to shock the pool.

Use it at night or in the evening to protect free chlorine from UV light from the sun.

Also important, you need to wait for 8 hours before using the pool when you use calcium hypochlorite to shock a pool.

2. Lithium Hypochlorite Pool Shock

Lithium hypochlorite has the lowest percentage of chlorine (35%) and the most expensive type of pool shock available in the market.

To use lithium hypochlorite, you don’t need to pre-dissolve it like calcium hypochlorite needs.

Use it at night or in the evening to protect free chlorine from the direct sunlight.

Lithium hypochlorite pool shock also needs that you wait for 8 hours before swimming in the pool.

3. Di-Chlor (Granular Chlorine) Pool Shock

Di-Chlor or granular chlorine pool shock has 60% chlorine concentration. This pool shock adds Cyanuric acid into the pool water and should be used with controlled amounts of Cyanuric acid in the pool.

Di-Chlor doesn’t need to be pre-dissolved in order to use it as a shock. Add it at night or in the evening to protect free chlorine from UV light from the sun. Wait for 8 hours before swimming in the pool.

4. Pottasium Peroxymonosulfate (Non Chlorine Pool Shock)h

Pottasium peroxymonosulfate is a nonchlorine pool shock, used mainly in Bromine swimming pools. However, it can also be successfully used in chlorine based pools.

Pottasium peroxymonosulfate doesn’t need to be pre-dissolved in order to be used as a pool shock.

On the positive side; you can use a swimming pool shocked with potassium peroxymonosulfate after only 15 minutes, unlike the rest pool shocks that have to be free of use for 8 hours.

Also, this pool shock can be added into the pool at any time as it is chlorine-free and cannot be affected by direct sunlight like the rest.

Which Type of Swimming Pool do you Manage

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How to Shock a Swimming Pool

Shocking or super chlorinating a swimming pool is not difficult and anyone can do it right when correct directives are followed. Here is an outline of all you need to do when shocking a swimming pool:

  • Prepare shock chemicals as required: depending on the shock chemical you use in your pool, some shock chemicals need to be dissolved before using in the pool. More details of 4 types of shock chemicals available in the market are discussed above.
  • Fill a bucket with 5 gallons of water from the swimming pool, which is about or equal to 19 liters of water.
  • Add the prepared pool shock into the bucket of water: Always don't add water to the pool shock, but the other way round.
  • Stir the bucket well to mix the pool shock with water.
  • Pour the bucket of dissolved shock slowly around your pool.
  • Refill the bucket with water and stir again to dissolve left over shock chemical in the bucket and pour slowly again around the pool.
  • Leave the pool for recommended hours before swimming depending on the type of shock chemical you use.
  • Measure the amounts of chlorine to make sure it is 3 ppm or slightly less before swimming in the pool as it is dangerous to swim in a pool with high chlorine concentration. You can use chlorine reduction reagents when necessary.

How to Shock a Swimming Pool


As we have seen, to have a healthy pool water free of bacteria, algae, and other organisms; there is no specific time frame set that is appropriate to shock a non-saltwater pool: Rather, how frequent you need to shock your pool depends on a number of factors discussed above based on which comes first. On the other hand, make sure that you do enough research on pool shocks, to ensure that you use the best type chlorine shock in your pool to avoid extra expenses that come with clearing contaminated, cloudy, green, or bacteria infected pool water.

Related Articles:

Clearing a Cloudy Swimming Pool Water

Clearing a Green or Black Swimming Pool Water

Swimming Pool Filters: Comparisons and Reviews

Lamotte ColorQ Pro 7: Pool Chemistry Made Easy

Sources of the Information

All the information included in this post are based on knowledge from my profession as a chemical engineer, personal working experience of more than 7 years, and standard rule and regulations by relevant bodies


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