When to Shock a Pool – 8 Signs You Need To Shock




When to use pool shock falls into two separate sections – under what circumstances most pool owners should shock a pool and, if it needs to be shocked, what is the best time of day to carry out pool shocking.

As well as regular pool shocking, you will need to shock the pool when opening it, when the weather is extremely hot, when it has rained heavily, if you have an algae attack or if the combined chlorine level is high.

There are many intricate things about shocking a swimming pool that a pool owner will need to understand. To always have a clean pool, you need to understand why a pool needs to be shocked and do it correctly each time.

when to shock your pool

When to add pool shock?

For regular maintenance, pools should often be shocked once every one to two weeks but this does depend on the water’s clarity and how much use the pool gets.

However, there are other times when it’s a good idea to shock chlorine pools outside of scheduled maintenance.

Here are a few other scenarios where you might find it a good idea to shock your pool: 

Opening up in spring

When you open your pool for the swimming season, spring algae has a chance to start propagating quickly with rising temperatures and exposure to sunlight. Shocking the pool with chemicals at this point can help prevent a lot of hassle and clean-up later.

After heavy use

If a high number of active swimmers use the pool, perhaps after a pool party, it can cause a drop in chlorine levels and the introduction of bacteria that can make the pool dirty. Shocking after parties can help bring the pool’s pH levels back to baseline and help sanitize it after heavy use.

Many people using it can mean that many oils, creams, and other products will be brought into your pool water. Using chlorinated shock will help eliminate these and make the water safe for swimming again.

Going on vacation

If you go on vacation for extended periods then giving your pool a good dose of unstabilized chlorine shock immediately before you go will help to prevent you from returning home to a nasty swimming pool.

Reducing Chloramines

Shocking is also a good idea if swimmers find their eyes and skin irritated by chloramines, which build up in the water as chlorine binds with ammonia-based contaminants like sweat and urine to form combined chlorine.

Once combined with other contaminants the chlorine is no longer free to deal with other bacteria etc (Source: Center for Disease Control)

Many swimmers mistakenly associate a strong chlorine smell or burning eyes with excess chlorine in the pool water, but as stated it means there is no free chlorine, only combined chlorine. 

A strong chlorine smell in pool water and eye irritation results from the chloramines in the water, not the chlorine. Adding additional chlorine to the water in the form of pool shock helps to reduce the smell and effects of chloramines as well as giving higher free chlorine levels. Adding non-chlorine shock will not fix this.

After heavy rain

Rain can change the pH levels of a pool while also introducing organic contaminants that encourage the growth of algae and bacteria.

Shocking the pool after rain can prevent the pool from turning green after heavy rains.

During periods of dry, hot weather

Hot sunny weather raises the pool water temperature, making it a more favorable environment for both algae and bacteria. Flushing the pool with fresh cold water to lower the warm water temperature and then shocking it can stop the pool water look cloudy.

Outside of regular maintenance, shocking a pool is a good preventative measure any time environmental factors increase the risks of bacteria and algae growth or disrupt the pool’s pH levels. 

Extreme algae growth

Using pool shock to increase the pool’s chlorine level significantly (known as super-chlorinating) will be the most effective way of fighting an algae attack. You may have to do this a number of times if it is a serious algae attack.

Algaecides are generally used as a preventative measure rather than as a way to deal with such an attack.

Unstable chlorine levels

If your chlorine levels never stand still, being too high one day and too low the next, a shock treatment may be needed. This shock will release most of the combined chlorine in your pool while re-balancing everything to easily allow the chemicals you have added to do their work.

Always test the pool’s water with a good test kit and shock if necessary.

It is important to understand the difference between free chlorine, total chlorine and the combined chlorine level so I suggest you also read my post – Free chlorine vs total chlorine vs combined chlorine explained

Also read: How often should you shock your pool

The best time of day to shock pool

If you shock a pool in the morning or during the day then the ultraviolet sunlight reduces the effectiveness of chlorine and makes it dissipate very quickly before it has a chance to work.

The best time of day to shock your pool is in the evening after the sun is down. This allows the pool chlorine time to spread out in the water and clean it before daytime temperatures reduce the effectiveness of the shock and dissipate it.

Shocking the pool in the evening also allows you the opportunity to run the pool pump overnight to help distribute the shock around the pool and this means that the pool will probably be safe to swim after shocking again the following day.

You should test the free chlorine levels to ensure that they have reduced enough to allow swimmers to return to the pool.

My article Can I swim 12 hours after shocking pool explains this in more detail.

when to add pool shock
Adding packet shock to a pool

How to Shock a Pool

Now that you know under what circumstances you should shock your pool, this is a quick guide to how to shock your pool.

The process of shocking your pool will soon become a habit that you can easily do.

Shocking your pool involves a few easy steps to follow:

1. Test the water

You should be testing the water every day with a strip anyway, but before shocking, we need to know what levels the chemicals in the pool are at. Doing a full test with all the required chemicals will allow you to know exactly what to add.

2. Adding chemicals

Depending on the chemical readings you have just taken, you will know what chemicals you should add to the pool. It may require adding acid if the pH is high or adding baking soda or soda ash if the pH is too low first before adding shock. Hopefully, it will not require anything at all. Whatever it is, the chemicals must be balanced before shocking.

3. Wait until sunset

The most important part about adding a shock treatment to your pool is making sure the sun cannot destroy the chlorine. We recommend that you add a shock treatment just after sunset when the pool will not be hit by the sun anymore.

4. Add the shock

Depending on the type of shock (read the label) you can either add it directly into the pool, or mix it in a bucket filled with water first before adding it to the pool. I always walk around the side of the pool adding a little as I go to ensure it is distributed as thoroughly as possible.

IMPORTANT: Whatever you do, if mixing the shock and water in a bucket, always fill the bucket with water first and then add the shock to the water. Never add the shock first and then add water.

5. Run the pump overnight

This is one of the main reasons I leave it until the evening to shock my pool. I can then set the pool pump to run overnight to thoroughly mix the shock and allow it to do its “magic” when the sun isn’t shining on it.

The added bonus is that swimming after shocking the pool might then be able to recommence during the next day if the chlorine readings are safe.

6. Test again

Once everything has mixed in and the recommended time on the back of the shock treatment kit has passed, you must test the water again.

If everything is correct such as the chlorine level and the pH then that is it – job done.

If not, you may need to shock again, and this may also be the case if you add shock to kill algae but the pool water is still green.

Pool Care Handbook and Video Course

When I bought my house with a swimming pool, I knew absolutely nothing about pool care. I just winged it for a while, making many mistakes along the way.

Fortunately, I was recommended Swim University’s Pool Care Handbook and Video Course. I bought it and it was an absolute game-changer.

It was the best money I spent that year. I learned everything from basic cleaning to advanced troubleshooting. Swim University offers a no-quibble refund policy too so what do you have to lose?


Shocking swimming pools is a normal part of maintaining it and keeping it clean throughout the year and it also helps eliminate algae growth. Make sure you always have shock ready to use as you may need to do it any time to keep the pool sparkling.

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