How Do You Know When to Shock a Pool?

When to shock a pool falls into two separate sections – under what circumstances should you shock a pool and, if it needs to be shocked, the best time of day to shock a pool.

You should shock the pool after being unused for several months, when the weather has been extremely hot, or when it has rained a lot. If your chemicals are unbalanced or you see excessive algae growth, you should get the water ready to be shocked again to ensure a clean, sparkling pool.

There are many intricate things about shocking a pool that you will need to understand, with many people assuming that the process will always be the same. To have an always clean pool, you need to understand why a pool is being shocked and properly do it each time.

When to Shock a Pool

What are the triggers to shock a pool?

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 your pool 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

When many swimmers enter the pool at once, this 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 the pool often mean that many oils, creams, and other products will be brought into your pool water. Shocking will help make the water safe for swimming again.

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. (Source: Center for Disease Control)

Many swimmers mistakenly associate a strong chlorine smell or burning eyes with excess chlorine in the pool water, but the opposite is true. 

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 shock helps to reduce the smell and effects of chloramines. 

After heavy rain

Rain can interrupt the pH levels of a pool while also introducing 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 weather raises the pool water temperature, making it a more favorable environment for both algae and bacteria. Flushing the pool with fresh cold water and shocking it can keep hot weather from making 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

A shock kit will be the start of your fight against an uncontrollable number of algae that you may have to face any time during the year. The boost in chlorine mixed in with the other chemicals that your pool will experience during this time will comfortably have the algae dying out.

You must remember this when you are trying to find an effective way to kill all the algae that may be growing throughout your pool. Algaecides are only used in the most extreme cases as they will drastically affect the overall balance of the other chemicals in your pool.

Unstable chlorine levels

If your chlorine levels can 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.

Many people assume that stabilizers will comfortably have their pool become more chlorine stable; while this is true, a good shock kit will help further. The stabilizer will only assist the free chlorine and acid in the pool to be protected, while the shock kit physically boosts these chemicals.

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

The best time to shock a pool

The best time of day to shock a 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 day-time temperatures reduce the effectiveness of the shock. Ultraviolet light reduces the effectiveness of chlorine. 

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 also this means that the pool will probably be safe to swim after shocking again the following day.

You should ensure that the chlorine levels have reduced enough to allow swimmers to return to the pool. My article How long after shocking a pool can you swim explains this in more detail.

How to Shock a Pool
Adding packet shock to a pool

How to Shock a Pool

Now that you know when to shock your pool, we need to quickly look at just how exactly you can shock your pool regularly. The process of shocking the pool should become a habit that you follow every three to four weeks as it will make your life a lot easier to do.

Many month-to-month care kits automatically include a shock treatment at the end or the start of each month. Shocking your pool is only a few easy steps that you need to follow, but doing even one wrong or at the wrong time will mean that your shock has failed, and you need to start all over.

  1. Testing 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 several liters of acid, or it may not require anything at all; whichever way it is, the chemicals must be balanced before shocking.
  3. Waiting 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.
  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. Testing 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 normal 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 shocked to kill algae but the pool water is still green.

Pool Maintenance Course

When I first bought my house with a swimming pool I knew nothing about how to clean and maintain it. I was recommended Swim University’s Pool Care Handbook and video course so I bought it and have never regretted it.

It was probably the best money I spent that year as I have saved thousands by doing it myself.

Pool care handbook


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

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.