How to Shock Pool or Spa and Important things to Know Handy

Last Updated: | By Barack James

How to Shock a Swimming Pool

What is Shocking a Swimming Pool or Spa?

Shocking, sanitizing, or chlorinating a swimming pool or spa is a regular process of adding chlorine to your pool or spa water, to raise and maintain free chlorine (FC) at the recommended level of 3ppm for a swimming pool and 3-6ppm for a hot tub.

Shocking a swimming pool can also mean SLAM (Shock Level and Maintain), which is adding chlorine in your pool and maintaining very high FC levels between 12ppm to 24ppm for a non-salt water pool and 28ppm to 31ppm for a saltwater pool, to kill algae or ammonia that come as a result of poor sanitization of pool and spa. 

Adding chlorine in your pool or spa on a daily or weekly basis is vital since chlorine sanitizes your water by eliminating harmful germs and bacteria causing algae and ammonia in swimming pools.

Chlorine also eliminates chloramine or combined chlorine (CC), which is formed as FC level (FCL) in your water depletes due to 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 or spa water to keep away harmful bacteria and germs.

If you don't have a saltwater chlorine generator, 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 to maintain recommended FCL in your pool or spa.

For a saltwater pool, you don't need to add conventional chlorine manually because Saltwater Chlorine Generator (SWCG) does it for you, however, if you have cloudy water in your Saltwater pool or spa, visit how to maintain a Saltwater pool before opting to shut down your chlorine generator to shock your pool manually using chlorine.

Also, if you are running a spa and wondering how to sanitize it, we prefer using chlorine because your hot tub chemical needs will be less, than when you use non-chlorine oxidizers like bromine that will still need the use of chlorine to work effectively.

You can later check out a spa or hot tub maintenance guide to know more on how to maintain your spa at a very minimal cost, the best sanitizer to use in your hot tub and more on spa maintenance, for now, let's do the shocking.

Before we go into how to shock pool or spa to 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, you can grab your chlorine shock and scroll down to " 3 Easy Steps for Shocking Pool or Spa Using Chlorine" 

After How Long to Shock My Swimming Pool and Spa?

A good and recommended practice is to test your FC level every evening and raise it back to recommended levels if lower. Remember that the hot water in a spa will make hot tub chemicals work and deplete faster than a pool without a water heater. 

What duration to shock your pool or spa will largely depend on the type of chlorine you are using to shock your pool, and they include granular, liquid, trichlor, or dichlor.

If you use liquid chlorine(sodium hypochlorite), you will shock your pool or spa at least once a day if the pool or spa is busy.

If you use granular chlorine(calcium hypochlorite), trichlor, and dichlor, you will shock your pool every week.

How to Know Free Chlorine Level is Low 

Chemically balanced pool and spa 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 and spa. 

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

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 ultraviolet (UV) light from the Sun consumes chlorine faster and if you do it during daylight your FC will reduce very fast and leave your pool vulnerable to harmful germs and bacteria.  

Two Things to Consider Before Shocking Your Swimming Pool

  1. Which is the Best Chlorine Shock?

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

1. 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% available chlorine against 5% to 8% available chlorine found in regular bleach.

Sodium hypochlorite is recommended for daily sanitization of pool water because it does not deposit other chemicals in your water including Calcium and Cyanuric acid.

Sodium hypo has a low acid ratio compared to dichlor and trichlor and therefore doesn't lower pH and alkalinity levels like granular, dichlor, and trichlor.

Liquid chlorine is also more effective since it mixes faster with water to kill bacteria and germs causing algae and ammonia.

Where Can I Buy Liquid Chlorine?

There are several liquid chlorine in the market today but most of them are below the standards needed to sanitize pool and hot tub water without water chemistry drama. 

If you live in the USA and asking where you can get the best liquid chlorine for your pool or spa, visit HASA Sani-Clor website.

Alternatively, you can physically 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, non stabilized, and stronger with 12.5% chlorine (Sodium Hypochlorite).

If you cannot get HASA Sani-Clor liquid chlorine, you can use the best alternative brands made of Sodium Hypochlorite like Pool Shock Liquid Chlorinator coming with 12.5% available chlorine, or KemTek chlorinating liquid that is 10% available chlorine and Pool Essentials Chlorinating Liquid that is also 10% available chlorine.

2Granular Chlorine: Also known as Calcium Hypochlorite, granular chlorine is the most common pool shock, I suppose because most pool owners don't prefer daily sanitization.

Calcium Hypochlorite is always 40% to 78% available chlorine and comes with high Calcium levels and using it needs a close watch on Calcium Hardness (CH) levels in your water, otherwise, you will fight the unending calcium scaling problem.

3Dichlor (Stabilized Chlorine): Dichlor comes with around 56% available chlorine and always best for startup spa or hot tub before shifting to unstabilized liquid chlorine.

Dichlor is always the best recommendation for fresh refills because it has a fair percentage of available chlorine and cyanuric acid that will not damage the pool or spa easily as compared to a much stronger trichlor.

After attaining Cyanuric acid level 30ppm to 40ppm, you need to shift from using dichlor to unstabilized liquid chlorine.

 4. Trichlor (stabilized chlorine): Trichlor is the strongest chlorine shock with around 80% to 90% available chlorine and always in a tablet form.

Trichlor is a stabilized chlorine and comes with high Cyanuric acid(chlorine stabilizer) levels. Trichlor might be an ideal option for fresh pool and spa water that need high initial FC and Cyanuric acid levels, and maybe when your pool or spa is attacked by algae or ammonia and you need very high free chlorine levels to kill them.

Otherwise, I always try to avoid trichlor due to high levels of available chlorine and stabilizer, which are both dangerous and unhealthy to the pool and its users. Very high FC levels might cause surface damage to your pool or spa.

You can use tablet chlorine as a start-up shock but later on, shift to unstabilized liquid chlorine when Cyanuric acid level reaches 30ppm - 40ppm.

Pools and spas maintained for a long time using stabilized chlorine are likely to be affected by high Cyanuric acid (Cya) levels if proper care isn't taken in balancing chlorine stabilizer.

The downside of having excess Cyanuric acid level is that to reduce its level, you have to partially drain your pool or spa water and refill it with fresh water. 

I guess this information will guide your choice of the best chlorine shock to use in your pool or spa, but we highly recommend you adopt the use of liquid chlorine to sanitize your pool and hot tub.

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

Cyanuric acid(Cya) is a chlorine stabilizer and its 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 always be 3ppm with a cyanuric acid level of 30 to 60ppm for non-salt water pool and spa, 70 to 80ppm for a saltwater pool and spa, and 20-30ppm for indoor pool and spa. 

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 to your water because the only way to reduce its level is by draining and refilling a portion of your pool and spa water. 

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 in your pool or spa.

3 Easy Steps for Shocking Pool and Spa Using Chlorine

  1. Measure The Correct Dose for Liquid, Granular, and Stabilized Chlorine 

Chlorine Dosage: The General Measurement:

1lb/1pound = oz. × 0.062500 

  • Sodium Hypochlorite(liquid chlorine): 10 fluid ounces(oz.) of liquid chlorine with 12.5% Chlorine (i.e HASA Sani-Clor Liquid Chlorine) raises FC level by 1ppm in a 10,000 gallons pool or spa.13 fluid ounces(oz) of liquid chlorine with 10% chlorine (i.e KemTek Chlorinating Liquid) raises FC level by 1ppm in a 10,000 gallons pool pool or spa.
  • Calcium Hypochlorite(Granular): 2 oz. of calcium hypo with 67% chlorine raises FC level by 1ppm in a 10,000 gallons pool or spa.
  • Trichlor /Tablet Chlorine: 1.5 oz. of trichlor raises FC level by 1ppm in a 10,000 gallons pool or spa.
  • Dichlor: 6.5 oz of Granular chlorine with 56% available chlorine raises FC level by 3.0 ppm in 10,000 gallons pool or spa.

2. Getting Ready to Shock Your Swimming Pool and Spa 

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 chlorine shock your pool or spa needs. Recommended range is 3ppm for non-saltwater and 3 - 4ppm for saltwater pool and spa. 

2). Test and know your Cyanuric acid level in the water, the correct range for non-saltwater is between 30 - 60ppm with the ideal level at 40ppm, 70 - 80ppm for saltwater, and 20-30ppm for indoor pool and spa. The 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 long after adding chlorine in your pool or spa, the initial chlorine dose when clearing cloudy water should be enough to raise FC level slightly above the recommended levels. For non-salt water, 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.

4). Since we recommend liquid chlorine pool shock, maintain FC level at the recommended levels 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. 

3. How to Add Liquid Chlorine, Granular, or Stabilized Chlorine in Your Pool and Spa

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

2). For granular 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 it to the pool or spa 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 into the pool.

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

Following this guide will raise your free chlorine level and clear cloudy water. Remember to maintain recommended FC and Cya levels all the time, daily or weekly depending on which chlorine shock you decide to use. 

If you put off your saltwater generator, make sure that FCL has settled between 3 - 4ppm before putting it back on, and remember to reset both the SWCG percentage setting and your pump run-time as appropriate to maintain the 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 are more detailed causes of cloudy pool water and more on how to clear stubborn cloudy swimming pool water.

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