Pool Shock: Everything You Need to Know

Pool Shock: Everything You Need To Know

The first thing that comes to anyone’s mind when they think of a pool is a deep tank brimming with crystal clear blue water and cheerful swimmers thrashing around.

But that full of life image of a swimming pool necessitates a proper cleaning regimen; otherwise, it becomes the perfect breeding ground for various disease-causing bacteria and nasty pathogens that make swimmers sick.

Stats by the Centre of Disease Control show that the rate of contracting viruses has significantly shot up since 1978, leading up to 879 disease outbreaks due to recreational waters.

And over the observed four decades, 500 out of the 879 breakouts occurred from 2000 to 2014.

To combat the onslaught of waterborne microbes, pool chemists and caretakers use chlorine or bromine to treat pool water. It disinfects the water and balances pool chemistry, aka the chemical levels.

Proper pool chemistry entails balancing four chemical components in the water to keep a pool safe for swimmers.

Whenever any of these constitutes are not within the ideal range, which happens more often than not in pool water, restorative measures must be taken that involve adding water stabilizers to counteract the increased or decreased value of the respective component.

ChemicalsIdeal RangeWhen HigherWhen Lower
pH7.2-7.8Add a pH decreaserAdd a pH increaser
Calcium hardness150 – 400 parts per million (ppm)-source water has high hardness, use a mineral removal agent (aka Stain and Scale, mineral sequestering agent) -source water has low hardness, “dilution is the solution” (i.e., drain some of the water and add clear freshwater)Add Calcium Hardness Increaser
Total alkalinity80 – 120 parts per million (ppm)Add a pH decreaser (or muriatic acid)Add a Total Alkalinity Increaser
Cyanuric acid20 – 100 ppmDilution is the solution, aka adding more waterAdd Cyanuric Acid (aka water stabilizer)

Maintaining pool chemistry is vital for sanitization measures to take effect; otherwise, no matter how much chlorine is added to the water, disinfection will most likely not take place as it should.

Speaking of adding chlorine to pool water, it is done in two ways or, more accurately, uses two forms of the compound.

Many people assume that chlorination of water is a linear process that involves adding chlorine any which way. But that’s not true. While essentially chlorination is mixing the antiseptic compound in water, it is a little more elaborate in how it’s done and what form of chlorine is used.

There are two types of chlorine used for treating pool water; stabilized and unstabilized. Stabilized chlorine comes in granular form or tablets and is added in small amounts for daily sanitization using automated chlorinators.

In comparison, unstabilized chlorine is used for weekly chlorination in which high doses of the element are added to the water-this process of using large amounts of chlorine to shock the pool is known as pool shocking or giving a shock to the pool.

Pool shocking is proven to be more effective in providing pool water with powerful chlorine concentration that keeps the water fresh, free of algae, and other organic contaminants.  

What Does Pool Shock Do?

What Does Pool Shock Do?

The term pool shock is used to describe the phenomenon of mixing high concentrations of chlorine in pool water to sanitize balance the water attributes and disinfect the tank to create a healthy swimming environment.

Since a high dose of the oxidizing compound is added to the pool water, the procedure is referred to as pool shocking.

Pool shocking or the intentional over-chlorination of pool water is also known as super-chlorinating/chlorination, and it is only done for a specified amount of time.

Super chlorination is an essential pool maintenance protocol that impedes the growth of disease-causing bacteria and viruses and rids a pool of debris and other organic waste that standard chlorinating doesn’t achieve.

The regular residual amount of chlorine added to pool water is not enough to keep gunk and pathogens from making abode in the pool tank because it is consumed by the floating organic waste, such as body oils, urine, and lotions.

As a result, the added disinfectant doesn’t do much in cleaning the pool.

Therefore, pool shocking is vital for a germ-free, healthy swimming environment as the procedure feeds excessive chlorine into the pool that is enough to satisfy the chlorine need of organic matter and sanitize the water.

Chlorine in Water

Once chlorine is added to pool water, it takes up any of the following forms

  1. Free available chlorine (FAC): the molecules of chlorine that do not react with organic contaminants and are free to go about killing pathogens.
  2. Combined available chlorine or Chloramines: the molecules of chlorine that react with organic matter such as sweat, urine, and body oils.

FAC and CAC combine to form total free chlorine in pool water.

When a pool is shocked, it’s free chlorine count is increased by shooting up the quantity of free available chlorine molecules. While all pool shocks are supposed to ramp up chlorine concentration in water, they are all not prepared the same way.

That is, some pool shocks are made with a particular chlorine compound, while others are created by a different variety of chlorine powder.

Typically, two types of chlorine shocks are used in both residential and commercial pools. But there is another form of pool shock that doesn’t use chlorine.

Here is a more in-depth look at the three forms of pool shocks:

Calcium Hypochlorite

Also referred to as cal hypo, calcium hypochlorite is the most commonly used chlorine shock in the pool industry. It is considered the most potent of all chlorine shocks that also dissolves the fastest in pool water.

Both a sanitizer and oxidizer, calcium hypochlorite, can increase the pH of pool water; therefore, using a counteractive agent to balance the disrupted value is recommended.

Moreover, cal hypo can spike the level of calcium hardness; hence, a decreaser must be used to restore the balance. 

Dichloroisocyanuric Acid

As evident by the name, dichloroisocyanuric acid contains cyanuric acid along with chlorine and is sometimes known as dichlor.

Like calcium hypochlorite, this chlorine pool shock is both an oxidizer and sanitizer that can increase the cyanuric acid concentration in pool water, thereby lowering its pH level.

Sodium Monopersulfate

Sodium monopersulfate is a non-chlorine alternative to chlorine shocks. It is also known as MPS shock in the pool industry. It is an oxidizer but not a sanitizer, which means it clears out all the organic matter and waste without thoroughly disinfecting the water. 

Even though all the information provided in this article thus far is enough to attain a clear understanding of pool shocking, one misconception regarding the topic needs to be addressed in a little more detail. And that is the relationship between chlorine and shock.

Are Chlorine And Shock The Same Thing?

Since shock contains chlorine, the two concepts are often considered the same and used interchangeably. However, they are different. While a pool shock uses chlorine, it is not entirely a chlorine sanitizer used for chlorination.

Instead, it’s a highly concentrated, powerful chemical formula containing chlorine that oxidizes and sanitizes pool water while increasing the free chlorine content.

On the contrary, chlorine or a regular chlorine sanitizer, though chlorine-based, is a much more diluted mixture that is added to a pool.

Aside from the strength of the two compounds, how they are used to treat pool water differs. Generally, when chlorinating water, a 3-inch granular tablet is used, that maintains the chlorine level in water but doesn’t necessarily eliminate robust pathogens.

The chlorine tablets used for daily chlorinating are not sufficient for pool shocking as the latter requires hefty amounts of the compound to achieve results.

When shocking a pool, the formula used should be potent enough to raise the free chlorine level above 5 parts per million for several hours to eliminate contaminants.

Apart from the confusion regarding chlorine and a shock, another frequently asked question about the two pool maintenance mechanisms is whether a pool shock contains chlorine or not. Let’s dissect this topic in complete depth.

Does Pool Shock Contain Chlorine?

Does Pool Shock Contain Chlorine?

As mentioned above, pool shock primarily contains chlorine as the main idea behind the procedure is increasing the chlorine concentration in pool water. Therefore, a pool shock needs to have chlorine.

Typically, a granular oxidizer (or simply powdered chlorine) is used to shock a pool.  However, some caretakers find the smell of chlorine repulsive, so they prefer to use a non-chlorine alternative to shock their pools.

Moreover, chlorine can have a slew of harmful effects on bathers, such as itchy eyes, nausea, and vomiting, dull chest pain, burning sensation in the throat.

To save swimmers from developing ailments, people can use a non-chlorine alternative to oxidize the pool without the adverse repercussions. The most commonly used non-chlorine pool shock formula is Sodium Monopersulfate or MPS.

Using Chlorine Pool Shocks-Benefits

  • Quick-release and fast-acting
  • Oxidizes organic contaminants to purify water
  • Removes chloramines and ammonia
  • Kills algae and bacteria easily to disinfect water
  • The cheapest way to shock the pool (Cal Hypo)

Using Chlorine Pool Shocks-Drawbacks

  • High levels of chlorine can be harsh on soft and shiny surfaces
  • Cal Hypo adds calcium, and Dichlor adds cyanuric acid (‘Pro’ in some cases)
  • More hazardous to store and use, and has a strong odor
  • The residue of ‘shock dust’ is often left, or water becomes cloudy

Using Non-Chlorine Pool Shocks-Benefits

  • Quick-release, fast-acting, no residue, no odor
  • pH balanced at a near-neutral pH
  • Oxidizes organic contaminants to purify water
  • Removes chloramines and ammonia
  • No Pre-Dissolving needed; can be directly poured into a pool
  • Swim Immediately, no waiting period
  • Won’t bleach or fade vinyl liners or swimsuits
  • Won’t add calcium or cyanuric acid to the pool

Using Non-Chlorine Pool Shocks-Drawbacks

  • Not as useful for algae treatment
  • Not as useful for bacteria treatment

When Should I Shock My Pool?

Like every upkeep procedure, pool shocking has a protocol that must be followed to get the best results from the process.

That said, there is no scientific way of calculating the time to shock a pool; however, whenever you need to tackle the following contaminants or ensure that they do not inhabit your pool tank, you should shock your pool.


Algae is the sworn enemy of water that doesn’t only contaminate water but also turns it unsightly. Whether green, blue, yellow, or black, algae is a tough water contaminant to tackle.

Only intense chemical combinations can eradicate algae from pool water, and the best algaecide is chlorine but in excessive quantities.

Therefore, standard chlorination is not enough to treat algae. Although the severity of an algae bloom determines how much chlorine must be used for pool shocking, generally 10-30 ppm is required to exterminate the repulsive foreign matter.

Bacteria and Bather Waste

Since pools are used by swimmers, human waste can make its way into the water. Bather remains in a pool mostly include sweat, body oils, urine, hair, and lotion, among other things. Aside from human waste, bacteria can also blossom in pool water due to stagnation.

That is, whenever water is contained in one place for long durations, it develops a musty smell and pathogens. Therefore, pool water becomes home to both elements frequently, necessitating regular cleaning.

Chloramines, Contaminants, and Cloudy Water

Combined chlorine molecules can irritate the eyes and skin of swimmers in a pool.

Hence their level has to be maintained at a particular point, ideally 5.5 ppm. When the CAC value exceeds the 5.5 ppm mark, a chlorine or non-chlorine shock is required to break apart the radicals and secure balance.

Although these three pollutants are the main contributors in necessitating a pool shock, some other occasions can also warrant pool shocking. Those include

Reopening a Pool

When a pool is reopened after staying closed for the entire winter season, it should be shocked after the water balance is restored to kill bacteria and wipe out algae.

In some cases, caretakers shock their pool before closing it down to reduce the intensity of pollution during the dormancy period.

After Heavy Rainfall

Contrary to popular belief, rainwater can contaminate a pool because it carries air-borne particles.

Yes, initially, rainwater is pure, but as it makes its way towards the earth, it picks up pollutants from the environment that mix in pool water, destroying the pool hygiene. Therefore, shocking a pool after a spell of heavy rainfall is crucial.

After a Pool Party

Naturally, after rigorous use, every facility demands cleaning.

Likewise, when a large group of hyperactive swimmers uses a pool, it becomes rife with bather waste and bacteria. And so, it’s essential to shock your pool after a pool party or an event that led a flock of swimmers to your pond.

One caveat you must keep in mind when shocking a pool is that you must never do it when the sun is out. Why?

Because sunlight reacts with unstable chlorine and destroys it, essentially turning the entire procedure futile.

Always shock your pools in the evening when the sun is peacefully resting in the west. You will also find this piece of advice at the back of most high-quality store-bought pool shock packages.

Aside from all the mentioned ways to figure out the best window to shock your pool, as a general rule of thumb, do it once every week.

Can You Over Shock A Pool?

Many first-timers add a ton load of a shock to their pool when super chlorinating, assuming that the higher the amount of pool shock, the faster their pool will clear up.

But that’s nothing but a fallacy. The amount of chlorine added to a pool needs to be according to the size of the tank.

A quick method to work out the shock to water ratio is using 10 ounces of an oxidizing agent for every 10,000 gallons of pool water.  And if you are not sure about the amount of water in your pool, here is an easy formula.

Length of your pool (ft.) x depth (ft.) x 7.5 = volume in gallons.

If you follow the mentioned trick, you are most likely never to over-shock your pool.

However, if you do end up upending a bucket load of shock into your pool, you don’t have to worry about ruining the water quality or pool chemistry because there is no such thing as over-shocking a swimming pool.

If by over-shock, you mean simply adding more shock than the required amount, then yes, it’s possible that you put in an unnecessarily significant amount of product to shock your pool.

But that will not cause any irreversible damage. When excess chlorine is released in water, it burns offs on its own in a while. So if you mistakenly pour a lot of shock in your pool, just wait it out and let the sun dissipate it over time.


Chlorinating is only useful to keep a pool well-maintained, but if you wish to have a sparkling swimming pool and have a more elaborate cleaning regimen in place, pool shocking is the way to go.

It will eliminate all disease-causing microbes and create the healthiest and safest swimming environments for swimmers of all ages.

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