When and How to Shock Your Swimming Pool
What Is Shocking a Pool, and Why Should You Do It?
Shocking a pool (also known as super chlorination) is the process of adding an excess amount of chlorine to sanitize the pool—purging it of contaminants, and killing bacteria and other organisms in the water. This is a very important process in pool management that every pool owner should understand and know how to do.
Most people don't think about shocking until they have cloudy swimming pool water or pool algae. By regularly adding shock to a pool, you can avoid the hassle of clearing cloudy pool water altogether.
How to Shock a Swimming Pool
- Find out the volume of your pool. This will give you an idea of how much shock you need. The general recommendation is to use 1-2 pounds of shock for every 10,000 gallons of pool water.
- Make sure the pool water is at its normal level.
- Make sure your pool's pH is between 7.2–7.6, and alkalinity is between 80-120 ppm.
- Choose a shock treatment (chlorine or non-chlorine).
- Prepare the treatment according to the instructions on the package. Some types will require you to dissolve it in water first.
- If you need to dissolve the shock, fill a bucket with 5 gallons (19 liters) of warm water.
- Slowly add it to the water while gently stirring. Always add shock to the water—not the other way around. This makes it easier to dissolve.
- Pour evenly around your pool. If there is any undissolved shock left, add some pool water, mix gently, and pour it in the pool.
- Run the filtration pump for at least 24 hours to clear the contaminants from the pool. Shocking alone will only kill germs and algae; it won't get rid of them.
- If your pool is still green or cloudy, check that your chemical levels are adjusted correctly, scrub and remove any visible debris, and make sure your filtration system is working properly.
What Is Free Chlorine?
Unbound chlorine that is "free" to sanitize
How Long Do You Have to Wait to Swim After You Shock a Pool?
Before you can swim in the pool, wait for the amount of time recommended on the package—usually at least eight hours for chlorine-based shock treatments. You only have to wait as little as 15 minutes if you use non-chlorinated shock treatments.
To be safe, it's best to measure the amount of free chlorine in your pool to make sure it is 3 ppm or slightly less before swimming. It is dangerous to swim in a pool with high chlorine concentration. If necessary, you can use chlorine reduction reagents.
What Is the Best Time of Day to Shock a Swimming Pool?
Shock your pool late in the evening or at night, when the sun is down, to make sure the chlorine is doing its job. Ultraviolet (UV) rays from direct sunlight greatly reduces free chlorine levels, so shocking during the day will not be very effective.
If you absolutely must do it during daytime, use a chlorine stabilizer, such as cyanuric acid, to prolong the life the chlorine.
How Often Should I Shock My Pool?
How often you should shock your pool depends on how much the pool is used. For general maintenance and upkeep, every two weeks should be enough. Some people only do it once a month, although this isn't usually recommended. If people are swimming daily, you should shock every week.
When to Shock a Pool
Besides regular upkeep, there are five other times when you should shock your pool.
1. When Pool Water Temperature Rises
Bacteria and other organisms such as algae thrive in warm water. In addition, the amount of free chlorine decreases with rising temperatures.
Most pools are kept at a comfortable level of 86-88 °F. The temperature should be lowered to a range of 78-84 °F if there are a lot of swimmers or if the pool is used for athletic activities. You can use a pool thermometer to measure water temperature.
If the temperature rises above the recommended levels, you should shock your pool.
2. When the Free Chlorine Level Goes Below 3 ppm
The quantity of free chlorine in the water should be 3 ppm, which is also what the total chlorine level should be around. The combined chlorine level should always be maintained below 0.5 ppm—or at 0.0 ppm, if possible.
- Free chlorine is the chlorine content that is unused, or "free" to do its job: cleaning and disinfecting your pool.
- Combined chlorine, or chloramine, is the result of the chlorine's sanitizing action. Chloramine is formed from the combination of free chlorine and sweat, body oils, and urine, creating the infamous "pool smell" that people often attribute to chlorine. High levels of combined chlorine means there is less free chlorine to kill bacteria, parasites, and algae.
- Total chlorine is the sum of the free chlorine and combined chlorine contents.
Most test strips available on the market only measure free chlorine, but you also need to know values of total and/or combined chlorine (free and combined chlorine) before shocking your pool.
I use the digital pool water test kit. I like this equipment since it is accurate, easy to use, and measures other pool chemicals and properties such as pH, bromine, cyanuric acid, and calcium hardness. Lamotte ColorQ Pro 11
Important: Improper maintenance of pool chemistry—especially pH and cyanuric acid levels—changes the efficacy of chlorine, so make sure you use accurate equipment and frequently check your pool water's chemistry.
If you find that the free chlorine level is lower than 3 ppm, it is time to shock your pool.
3. After Heavy Rains
Although this isn't always be necessary, to be on the safe side, it is advisable to shock after heavy rain that can alter your pool's pH and add contaminants.
- Before shocking, cleaning, or adding any chemicals after heavy rains, make sure the water level is reduced to the normal volume.
- Leaves and other debris will likely be washed or blown into your pool. Clean them out with a large pool net before treatment.
- The pool's pH will be the measure that will likely change the most. However, when the rain is acidic and alkalinity levels are within the required range, all you may need to do is adjust the alkalinity level.
- Most important: Total alkalinity (TA) is very damaging when it is out of balance. Frequently check the TA level so that it does not exceed the recommended range of 80 ppm to 120 ppm. Here is how to lower total alkalinity. To raise your pool's alkalinity, you can use baking soda (sodium bicarbonate).
4. During Extended Periods of Hot Weather
When the weather is hot, the water levels will rise above the optimal temperature range of 78ºF–82ºF, making it easier for bacteria and algae to thrive. Additionally, UV rays from bright sunlight will decrease the amount of free chlorine in your pool.
Make it a routine to shock your pool more often in the summer, when the weather is hot. Use cyanuric acid to stabilize the free chlorine and prevent UV rays from consuming the chlorine at a higher rate.
5. When the Pool Is Used Heavily or Frequently
Chlorine levels reduce more quickly when many swimmers use a pool on a consistent basis. You should measure levels of free chlorine and chloramine after heavy swimming, especially in commercial or public pools, and shock the pool as required.
To find out what amount of free chlorine or any other chemical to add to a pool, I use this pool calculator (by Trouble Free Pool) to help me find the correct amount of chlorine to add. All you have to do is enter your chemical and pH readings.
Controlling the chlorine levels in saltwater pools is easier. All you need to do is raise the saltwater chlorine generator (SWCG) to boost free chlorine prior to and after heavy usage.
What Type of Shock Do I Use?
There are four types of shock treatment, and each has its pros and cons. The type of pool shock to use in your pool will depend on your pool's requirements, such as pH and mineral content.
Note that using poor-quality chlorine shock will always bring more problems, like white scum or foam inside the pool.
1. Cal Hypo (Calcium Hypochlorite)
- is the cheapest and most popular type of pool shock available on the market. Calcium hypochlorite
- It's also the strongest chlorine treatment, making it great for sanitizing your pool and killing algae.
- The high chlorine content of roughly 65% also means it will burn off quickly if used when the sun is still out.
- It has a high pH of about 11.7.
- It is an excellent choice if you need to raise your pool's calcium levels.
- It needs to be pre-dissolved before it is used.
- After treatment, you need to wait for eight hours before swimming, but check the manufacturer's instructions on the package to be safe.
2. Lithium Hypochlorite
- Lithium hypochlorite has the lowest percentage of chlorine (35%).
- It is more expensive than calcium hypochlorite, but it makes a good alternative if you don't want to alter your pool's calcium levels.
- It won't affect your pool's chemistry, and conversely, it won't be affected by pH or temperature, unlike other chlorine treatments.
- You don’t need to pre-dissolve it. Just add it straight into your pool.
- You should wait for eight hours before swimming in the pool.
3. Dichlor (Dichloroisocyanuric Acid)
- Dichlor, or granular chlorine, typically has around 60% chlorine, although lesser and higher concentrations are available.
- It has a neutral pH.
- It also contains cyanuric acid, which means it will raise your cyanuric acid levels, so do all of your chemical measurements before using dichlor.
- Although the cyanuric acid is meant to protect the chlorine from UV rays, use dichlor only when the sun goes down.
- It can be used for shock or daily chlorination.
- Doesn’t need to be pre-dissolved, so you can add it straight into your pool.
- Wait for eight hours after use before swimming.
4. Potassium Peroxymonosulfate (Non-Chlorine Pool Shock)
- is a non-chlorine shock treatment used mainly in bromine swimming pools. However, it can also be successfully used in chlorine-based systems. Potassium peroxymonosulfate
- It doesn’t need to be pre-dissolved.
- Because it doesn't contain chlorine, you can use it to use it at any time of the day.
- You only have to wait 15 minutes after use before swimming because it is non-chlorinated.
Shocking Saltwater vs. Chlorine Pools
Before I changed my non-saltwater (chlorine-based) pool to saltwater two years ago, I used to shock regularly.
Basically, a chlorine-based pool needs more maintenance than a saltwater pool. Unless there is an algae outbreak or a build-up of contaminants such as oil and soil, a saltwater pool does not need much treatment. This is because saltwater pools use chlorine generators to produce a chlorine compound similar to the chlorine in shock treatments.
Chlorine generators can be adjusted to increase the amount of chlorine in the pool, for instance, before heavy usage. However, this technically isn't shocking. It is just a way to maintain chlorine at the recommended level.
It may seem like there is too much to think about, but regular pool maintenance is a must if you want to avoid the headache of bigger messes to clean or expensive damage down the road.
Here are three things you should take away from this article:
- Shock your pool frequently.
- Do it at night.
- Wait at least eight hours before swimming.
Which Type of Swimming Pool do you Manage
How to Shock a Pool
Questions & Answers
Your article says to shock the pool if the free chlorine drops below 3 ppm. If the target is 3 ppm, the cL level will often be below 3 ppm. Why wouldn’t merely increasing the level a bit accomplish the task? What am I not understanding?