What Does Pool Shock Do? Pool Shock Facts

Every pool owner loves to see their family enjoying the water throughout the summer and just having fun. However, every pool owner quickly starts to dread the signs of algae, bacterial, and other growths in their pool, so turn to a shock treatment for help. But exactly what does pool shock do?

Pool shock increases the total amount of free chlorine in your pool, either by adding more chlorine or other shock chemicals that break apart combined chlorine. The increased level of free chlorine then destroys contaminants such as algae, bacteria and chloramines.

If all of the chlorine in a pool has combined with contaminants then there is effectively no chlorine available to kill algae etc. So in order to fix that you need to increase the amount of free chlorine in the pool by adding more or by “freeing” the chlorine that has become chemically locked.

What Does Pool Shock Do?
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Shock increases the free chlorine level

There are two ways that shock increases the free chlorine levels within a pool. The first way is by adding more chlorine to the pool. The second is by reacting to the stored, combined chlorine already in the pool’s water, releasing it.

This process is confusing as many people mistakenly think adding shock to the pool is simply adding more chlorine. This leads to many thinking that if they simply add chlorine from their own supply to a pool, they are effectively shocking the pool.

However, this is not true, and you can have two different types of shock treatments for your pool, depending on how high the current pH levels are. A higher pH level means a lot of combined chlorine in the pool, usually causing a strong chlorine smell in the pool.

What chemicals are in pool shock?

Calcium hypochlorite is the usually main ingredient of shock, with the mixture holding around 70% chlorine and calcium.

There are also shock packs with lithium hypochlorite that contains 35% chlorine and dissolves a lot faster than normal shock treatments would.

Further, you can also get dichlor, which many people mistakenly call liquid chlorine, when a mixture of chemicals contains 55% chlorine. This is often used because it does not need to dissolve into the pool water; it is already a liquid and starts reacting immediately.

The last type of pool shock contains no chlorine but oxygen instead and is only used if you want to go swimming soon after adding it to the pool. You can add these types of shock, wait 15 minutes, then safely go swimming in the water without having to stress about burns or rashes from the chlorine forming.

Types of pool shock

Types of pool shock - Chlorine or Oxygenizer

Different types of pool shock can make a difference to the time required before swimming so the first piece of advice advice would be to read the manufacturers instructions on the packaging. This should tell you how long after shocking a pool before swimming can recommence.

Chlorine shocks

There are two main types of Chlorine shock (that is, shock containing Chlorine) which are Calcium Hypochlorite (often referred to as Cal-Hypo) and Dichloroisocyanuric acid (often referred to as Di-Chlor).

Cal-Hypo is the one you will see for sale most often, primarily because it is not only the cheapest but strong too. It is very quick to dissolve (always in a bucket or container pre-filled with water before being added to the pool). A very popular Cal-Hypo shock available on Amazon is the In The Swim Chlorine Pool Shock which comes in 24 1 pound bags.

Di-Chlor is a granulated, stabilized chlorine. It is slower dissolving than Cal-Hypo and the stabilizer will raise the cyranuric acid level in your pool. In the Swim Sodium Di-Chlor Chlorine Shock is available in 24 x 1 pound bags on Amazon.

You can also buy a liquid chlorine shock which doesn’t require pre-mixing or dissolving.


Non-chlorine shocks

Non-chlorine shocks are known as Oxidizing shock. They contain no chlorine but instead uses oxygen to to remove contaminants from the pool. With this type of shock you can usually swim in as little as 20 minutes after adding it to your pool. However, Oxidizing shock will not get rid of algae so you will need chlorine shock for that.

One of the most popular on Amazon is the In The Swim Chlorine-Free Oxidizing Pool Shock which comes in 24 handy 1 pound bags.

Can you over shock a pool?

You can add too much shock to your pool; usually, this happens with new pool owners when they have to do their first pool shock as they add the shock and then start panicking when they see cloudiness in the water, seemingly nothing happening, or just testing constantly, It can all cause stress for pool owners.

If you add too much shock, the water can become dangerous as the chlorine reacts with other chemicals in the water. The calcium in the shock can cause buildup, while the pool’s chlorine levels can stay high for more than a week, turning the pool dangerous to be in.

As your pool chlorine levels continue to increase, you will find that the pool water can easily damage your skin or even cause your hair to turn green. Chlorine at high levels can cause a lot of damage to your body, making the pool you are swimming in dangerous.

Should you backwash after shocking?

If you have simply given the pool a regular shock treatment then you can backwash after vacuuming the pool once the chlorine levels have reduced back down to normal.

If you have backwashed due to an infestation of algae then you will probably have a fine layer of dead algae and bacteria at the bottom of the pool that must be vacuumed up. If that is the case then rather than vacuuming on filter and then backwashing, due to the shear amount of dead algae there will probably be, it would be better to vacuum to waste instead. That way the filter will not become clogged as all of the dead stuff will be ejected from the pool.

chlorine pool shock

How do the chemicals react differently?

Now that you know what is in the shock and how much of it each type of chemical can be held in each type of treatment, you need to know how they react. This is easily one of the most important parts about using a shock treatment, as most people only use them when a pool is green.

While shock treatments can help balance the chemical levels in a pool, they are usually only used when the pool is nearing a level that cannot be saved with normal means. There are five things that a shock treatment will react to, each one leaving behind different carcasses that need to be handled.

  • Algae: The free chlorine that the shock treatment creates will kill all the algae currently in your pool, no matter their size. This creates a soft, easy-to-disturb blanket of dead algae plants that sink to the bottom of your pool.
  • Plants: Depending on the type of plants that are in your pool, hopefully, nothing that is growing, the chlorine may not have any reaction. However, dead or alive leaves will all be bleached a bright white before turning into dust if left for long enough.
  • Murkiness: Depending on what is causing the murkiness, the shock can increase or decrease the total murkiness in the pool. A chemical imbalance might be made worse by shock treatment, while murkiness from algae will die out and sink to the bottom.
  • Bacteria: You won’t see the bacteria in the pool, but once the free chlorine levels in the pool increase, they will all die out. Chlorine does not leave anything behind, and bacteria are easily killed by the chemical once it is no longer stuck to other chemicals.
  • Pathogens: Pathogens can be harmless, good, or bad for you, and chlorine will not distinguish between them. The extremely high chlorine content in your pool will kill off any pathogens while the chlorine is present; however, as the levels drop, they return.

What time of day is best to shock a pool?

Ideally you should shock a pool in the evening, to ensure that the sun is not eating away at the chlorine in the pool before it has a chance to work. The sun is always the enemy of any pool owners as it causes chemical reactions with the water to bond free chlorine with other chemicals like nitrogen.

This is a natural process as the sun adds heat to the pool, making reactions a lot more possible and giving the free chlorine the chance to react with everything comfortably. We highly recommend that you ensure that the pool is not heated while doing this and having the water be its natural temperature.

Usually, this is why you will find that pools need to be shocked as the temperatures change as well, as the sunlight through the day lasts longer. Adding a shock treatment at night will mean that the pool is ready and willing to stop anything from growing when the sun does eventually start to rise.


Your pool may need to be shocked once a year to ensure that all the chemicals are nice and reacted with the right things. The shock is an excellent way to reset the current chemical levels contained in your pool, allowing you to enjoy it for much longer.

Whatever you do, please don’t swim in the pool after adding a shock treatment; no one wants copper green hair!