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How to Shock a Swimming Pool and Important things to Know Handy

How to Shock a Swimming Pool

What is Shocking a Swimming Pool?

Shocking or sanitizing or chlorinating a swimming pool is an important process of pool maintenance, which is adding chlorine to your pool water to raise free chlorine (FC) to recommended levels and eliminate chloramine or combined chlorine (CC), which is formed as FC level (FCL) in your water depletes due to heavy pool usage or as a result of UV light from the Sun. 

Combined chlorine turns pool water cloudy, smells chlorine, and cannot sanitize your pool water to keep harmful bacteria and germs causing algae and ammonia away from your pool.

If you have a non-saltwater pool, you need to add chlorine manually to your water at least once a day or a week depending on which chlorine shock you are using in order to maintain recommended FCL, which should always be 3ppm with Cynuric acid (Cya) ranging between 40 - 60ppm.

For a saltwater pool, you don't need to add conventional chlorine manually because Saltwater Chlorine Generator (SWCG) does it for you, and you only have to regulate the amount of chlorine that is introduced in to your water by adjusting the percentage setting in your saltwater generator up or down, where the higher the percentage, the more chlorine is produced and vise versa. 

The ideal FC level for a saltwater pool should be between 3 - 4ppm with Cyanuric level between 70 - 80ppm. If your saltwater pool is cloudy and your SWCG percentage is at extreme levels, you can raise your pumps run-time to enable more chlorine production.

However, if both the chlorine generator and pump has reached higher percentage and run-time levels respectively but your water is still cloudy with low FC level, you will have to shut down your saltwater generator and shock your pool by adding chlorine manually. 

This article is about sanitizing a swimming pool, which is adding chlorine to raise FC and attain the recommended free chlorine level to clear cloudy water. Sanitizing a pool doesn't need a lot of chlorine because you are just topping up to maintain ideal FC level.

However, if your pool has algae or ammonia, whether saltwater or non-saltwater, you will use a lot of chlorine since you need to attain higher FC shock level and maintain (SLAM) it at that shock level by adding more chlorine when FC drops at a given level and repeat until you get rid of algae or ammonia.

Before we go into how to shock a swimming pool to raise FCL and clear cloudy water, let's discuss few details you need to know before shocking the pool like when to shock, time to shock, and best chlorine shock to use.

Otherwise, skip to the bottom of this article to "how to shock your swimming pool" section and find out how to shock saltwater or non-saltwater pool using liquid, powder, or tablet chlorine.

After How Long to Shock My Swimming Pool?

A good and recommended practice is to test your FC level every evening and raise it back to 3ppm for non-saltwater pools, or adjust your SWCG percentage as appropriate for a saltwater pool and you won't have cloudy water or harmful bacteria causing algae and ammonia, which are expensive and time consuming to get rid of.

What duration to shock your non-saltwater pool will depend on which type of chlorine you are using to shock your pool, and later in this guide you will be able to choose which is best for your pool; powder, liquid, or tablet chlorine.

If you use liquid chlorine(sodium hypochlorite); you will shock your pool at least once a day. However, if you use powder (calcium hypochlorite) or tablet chlorine; you may need weekly shocking but with a lot of work in balancing pH, Calcium Hardness, and Cyanuric acid that increase when powder and tablet chlorine are used in pool water.

How to Know Free Chlorine Level is Low 

Chemically balanced pool water is the beginning of easy swimming pool maintenance, as such, you need a very accurate and reliable pool chemical test kit like LaMotte ColorQ Pro 7 that tests all chemicals in your water including FC, TC, pH, Total Alkalinity(TA), Cyanuric acid, Calcium Hardness, and Bromine for bromine maintained pools. 

The other way to know your FCL is low is when your water is cloudy and has strong chlorine smell, but you should not wait until your water is cloudy to shock your pool because you might have bacteria causing algae and ammonia.

The secret is to measure FC level in your water at least every evening for non-saltwater pools and adjust it by adding chlorine, or adjust your SWCG percentage setting for saltwater pools to raise your FC level.  

What is The Right Time to Shock a Swimming Pool?

The best time to shock your swimming pool is during the evening when the Sun is gone. This is because ultra violet (UV) light from the Sun consumes chlorine faster and if you do it during day light your FC will reduce very fast and leave your pool vulnerable for harmful germs and bacteria.  

Things to Consider Before Shocking Your Swimming Pool

1). Which is the Best Chlorine Shock?

This is very important. There are a number of chlorine shock available in the market and you need to know what type of chlorine shock you are using and their side effects. 

Liquid Chlorine: The first and most recommended chlorine shock is liquid chlorine also known as Sodium Hypochlorite. Liquid chlorine is similar to regular household bleach but much stronger with 10 to 12.5% free available chlorine against 8% chlorine found in regular bleach.

Sodium hypochlorite is recommended for daily sanitization of pool water because it does not deposit other pool chemicals in your water ending up raising their levels, it mixes faster with water and you don't wait for long to swim and faster in killing bacteria and germs causing algae, and ammonia.

Where Can I Buy Liquid Chlorine?

There are a number of liquid chlorine in the market today but most of them are below the standards needed to sanitize pool water without causing other pool problems. 

If you live in the USA and asking where you can get best liquid chlorine for your pool, visit HASA Sani-Clor website. Alternatively, you can phisically find HASA Sani-clor dealer near you and buy HASA Sani-Clor liquid chlorine, which is the best liquid chlorine I ever used because it's calcium-free, pH neutral, no stabilizer or Cyanuric acid and stronger with 12.5% chlorine (Sodium Hypochlorite).

If you cannot get HASA Sani-Clor liquid chlorine, you can use other best alternative brands made of Sodium Hypochlorite like Liquid Chlorine Pool Shock Commercial Grade by Pool Shock Liquid Chlorinator that is 12.5% chlorine concentrated, KemTek chlorinating liquid that is 10% chlorine concentrated, and finally Pool Essentials Chlorinating Liquid that is 10% chlorine concentrated and can be found at Walmart.

Powder Chlorine: Also known as Calcium Hypochlorite, powder chlorine is the most common pool shock. Calcium Hypochlorite is about 67% chlorine and comes with a high Calcium levels and using it close watch on your Calcium Hardness (CH) levels in your water otherwise you will definitely fight unending calcium scaling because of extreme calcium levels in your water.

Tablet Chlorine: Also know as Trichlor, tablet chlorine is the strongest chlorine shock with around 90% chlorine. Trichlor is mainly Trichloroisocyanuric and comes with high Cyanuric acid(chlorine stabilizer) levels. Pools maintained using tablet chlorine are likely to be affected by low pH and high Cyanuric acid (Cya) levels in the long run if proper care is not taken in balancing pH and Cya before adding chlorinated tablets. The down side of having excess Cyanuric acid level is that to reduce it's level, you have to partially drain your pool water and refill with fresh water. 

 Other Chlorine Shocks: Apart from the three common types of chlorine shocks discussed above, there are other two types but not very common in the market because of they are a little expensive. The first one Granular chlorine, also known as Dichlor. Granular chlorine are fast dissolving and it's the best option if you don't need to wait for long to swim after shocking your pool. The down side of Dichlor is that it raises your Cyanuric acid and lowers the pH levels like tablet chlorine.

The second other chlorine shock is Lithium Hypochlorite that comes in powder form. This chlorine shock is the most expensive and does not affect other chemicals level in your water like liquid chlorine.

I guess these information will guide your choice of the best chlorine shock to use in your pool. However, we highly recommend you adopt the use of liquid chlorine because it makes your work in balancing pool chemicals easier.

You can make your pool management work even easier by installing Saltwater Chlorine Generator (SWCG), which generates chlorine and dispenses the right amount of chlorine in your pool so that you don't have to add chlorine manually on a daily or weekly basis.

2). What is the Ideal Cyanuric Acid Level when Shocking my Pool?

Cyanuric acid(Cya) is chlorine stabilizer and it's level in your water will determine FC shock level, whether to clear cloudy water or to kill algae or ammonia.

The more chlorine stabilizer(Cya) in your water, the more chlorine you will need for the same effect because chlorine is weaker in high Cyanuric acid levels.

The ideal level of chlorine and Cyanuric acid will depend on why you are adding chlorine. For pool sanitization, FC reading should be 3ppm with cyanuric acid level of 30 to 60ppm for harsh chlorine(non saltwater) pools, and FC of 3 - 4ppm for saltwater pools with Cyanuric acid level between 70 to 80ppm.

However, to clear algae and ammonia in your pool, you may need higher FC shock level, in most cases ideal range is from 10ppm upto 24ppm for non-saltwater and upto 31ppm for saltwater depending on the intensity of algae and ammonia, and each FC shock level has its own recommended Cyanuric acid level.

Since reaching higher FC levels than recommended may damage your pool parts, be safe and make it a routine to ensure that your FCL is always stable within the recommended range to keep algae and ammonia away from your pool.  

Cyanuric acid is very important for your pool because it stabilizes chlorine and makes chlorine last longer in your pool. However, be very careful when adding Cyanuric acid in your water because the only way to reduce its level is by draining and refilling portion of your pool water. Keep Cya level between 20 - 30ppm for indoor pools. 

View this Chlorine/Cya chart by Trouble Free Pool to understand your chlorine shock levels and ideal Cyanuric acid level to clear cloudy water, or get rid of algae and ammonia.

How to Shock your Swimming Pool

The Correct Dose for Liquid, Powder, and Tablet Chlorine 

Chlorine Dosage: The General Measurement:

1lb/1pound = oz. × 0.062500 

  • Sodium Hypochlorite: 10 fluid ounces(oz.) of sodium hypo with around 12.5% Chlorine raises FC level by 1ppm in a 10,000 gallons pool.

  • Calcium Hypochlorite: 2 oz. of calcium hypo with 67% chlorine raises FC level by 1ppm in a 10,000 gallons pool.
  • Trichlor /Tablet Chlorine: 1.5 oz. of trichlor raises FC level by 1ppm in a 10,000 gallons pool.

Getting Ready to Shock your Swimming Pool

1). Shut your SWCG if you have one. Find out the volume of your pool in gallons and make sure water is at its normal level then take FC reading. Knowing your current FCL will give you an idea of how much shock your pool needs. Recommended range is 3ppm for non-saltwater and 3 - 4ppm for saltwater. 

2). Test and know your Cyanuric acid level in water, the correct range for non-saltwater is between 30 - 60ppm with ideal level at 40ppm, and 70 - 80ppm for saltwater pool. Cyanuric acid level in your water should always be the main factor to decide what amount of chlorine shock your pool needs. 

3). For FC level to be stable longer after adding chlorine in non-saltwater or saltwater pool, the initial chlorine dose when clearing cloudy water should be enough to raise FC level slightly above the recommended levels of 3ppm for non-salwater and 4ppm for saltwater pool. For non-saltwater, add enough chlorine to raise FCL between 5-7ppm, while for saltwater, add enough chlorine to raise FCL to 6ppm and wait till FC comes down to recommended levels for both and make sure you maintain FC at the recommended level all the time by measuring and adjusting FC every evening when it drops. View the Chlorine/Cya chart  and make sure you know what you are doing. 

How to Add Liquid, Granular, or Tablet Chlorine in your Pool

1). For Sodium hypo or liquid chlorine, you can add the right amount evenly around your pool or at the shallow end to raise FC level.  

2). For powder chlorine, make sure your pool's pH and Calcium Hardness are a little lower because they will also rise. Prepare the treatment according to the instructions on the package. Cal hypo may require that you dissolve it in water and mix well first before adding to the pool to get faster results. Pour Cal hypo evenly around your pool or at the shallow end. If there is any undissolved shock left, add some pool water, mix gently, and pour it in the pool.

3). For tablet chlorine, make sure Cyanuric acid level is a little lower because its level in water will rise. Remember that the only way to reduce Cyanuric acid is to drain and refill your pool water and you need to be very careful. To shock your pool, put chlorine tablet(s) at the skimmer.

Basically, following this guide will raise your free chlorine level and clear cloudy water. Remember to maintain recommended FC and Cya levels all the time.

If you put off your saltwater generator, make sure that FCL is between 3 - 4ppm before putting it back and remember to reset both SWCG percentage setting and your pump run-time as appropriate to maintain ideal FC level.  

Pool Still Cloudy After Shocking and FC is Balanced?

If you are done shocking your pool and it's still cloudy after 30 minutes with balanced FC, the problem could be something else and not low FC. Here is more detailed causes of cloudy pool water and more on how to clear stubborn cloudy swimming pool water.

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