When to Shock a Pool

While we’ve covered some of the basics about pool shock here at Sunplay, we have yet to go into some serious detail. We thought it might be time to answer some of the questions we get asked most often about pool shock, such as why, what, and when to shock a pool.

As you know, shock (or super chlorination) significantly raises the level of free chlorine in a pool in order to kill any organic material in the water via cell disruption. Chlorine oxidizes the contaminants that may have gotten into the water by attacking the cell walls of pathogens, bacteria, and residual organic matter in order to keep the water clear and sanitary. This process occurs as soon as liquid or granular chlorine is introduced into your pool.

If you add enough of the chlorine to the water (in a successful “shock” process), you will achieve what is called breakpoint chlorination, the threshold beyond which contaminants of any kind CANNOT survive.

While some recommend shocking the pool with chlorine once a month, you would do well to remember the ABCs of shock when considering whether or not to perform one on your pool water:

  • Algae
  • Bacteria
  • Chloramines/Cloudiness

Algae blooms can come in all kinds of colors and strengths. Whether it’s black, pink, or green, algae problems can be fixed with a chlorine shock. Whether it’s a small-scale bloom or algae across the entirety of the pool, go for a 30ppm chlorine level in order to eliminate the active algae. This works out to approximately 4 lbs of 65% calcium hypochlorite pool shock per 10,000 gallons or 3 gallons of 12% sodium hypochlorite (liquid shock) per 10,000 gallons.

If you have an algae problem, DON’T forget to also brush, vacuum, and clean out your filter to prevent it from recurring.


All kinds of bacterial growths from E.Coli, to Pseudomonas, to Legionella can grow in your pool. Yikes. All kinds of parasites, bacteria, and parasites can lead to illnesses for bathers which can potentially turn into hospital visits or even a rare fatality. Water is a great place for bacteria to breed in addition to being able to support viral and parasitic outbreaks. They get into the water by many means and then infect users via absorption, swallowing, or aspirating pool water.

Since you can’t see bacteria the same way you can see algae, you’ll need to test your water with bacteria test strips. These are quick, easy, and an inexpensive way to tell if you have a potential problem that can be easily treated with pool shock.


Chloramines are what are often to blame for your red eyes, itchy skin, and that powerful chlorine odor that happens around in pool water. Chloramines form when a chlorine molecule is floating around in the water looking to sanitize the water but instead, becomes combined with ammonia or nitrogen molecules. This renders it useless as a sanitizer and instead, creates unwanted side effects. These symptoms are often thought of as being due to an over-chlorinated pool when in fact, it needs MORE chlorine to correct the problem.

Chloramines can be measured with a DPD test kit. Subtracting free chlorine measures from total chlorine will give the level of combined chlorine in the water. If that number is 0.3 ppm or greater, you will need to shock the pool.

NOTE: In order to shock your pool correctly, you will need to achieve what is referred to as breakpoint chlorination threshold. You will need to raise the chlorine level 10 times the level of combined chlorine in order to not waste the pool shock.

Cloudiness can also be a significant problem in your water that can be helped by pool shocking, unless it has occurred because calcium or other suspended carbonates are the source of the cloudiness.

Oftentimes, cloudy water is caused by bad circulation, filtration, or sanitation issues. Shocking it may only be a Band-Aid on a problem that will need other attention in order to really be fixed. If you have recurring cloudiness, make sure your filter is not in need of replacement. However, cloudiness caused by a heavy bather load is a good idea in order to remove bather waste.


Like we said, most recommendations will be to shock your pool once every 30 days, whether it shows visible signs of needing it or not. Some sources will even suggest that you shock your pool once a week. Again, you should refer to the ABCs of pool shock and only shock in occurrences of algae, bacteria, chloramines or cloudy water.

If you don’t see any algae, you test for bacteria and there isn’t any indication that it’s living in the pool water, and chloramines aren’t present in the water, then you aren’t going to have to shock the pool. Don’t waste your money by treating the pool with expensive chemicals for conditions that aren’t present.

In addition to wasting chemicals, you could also damage plastic, vinyl, and steel liners that can become damaged in the presence of high levels of chlorine


To successfully shock your pool, you’ll need to execute a few specific steps to make sure you’re doing it correctly to get the most out of the treatment.

Remove Debris

If you have organic matter floating in the water or down on the bottom, that’s what the chlorine shock will attack. Instead of getting rid of one of your ABC problems, you will waste the strength of the shock. Use your pool skimmer and vacuum to make sure all of the dirt, bugs, and leaves are removed.

Lower the pH

Chlorine has to work much harder when the pH level is high, so lowering the pH in your pool (to around 7.2) will mean your chlorine’s effectiveness will be heightened.

Read the Directions/Label

There are all kinds of pool shocks available out there, all in different strengths and dosages.

You should also check the label to check how you should add the granules or liquid to the water—some can be added directly to your pool water while others may need to be dissolved in a separate bucket first.

Add to the Water

Add the chlorine shock throughout the pool surface while the pump runs. Be careful not to spill any on the surrounding deck or on your clothing. In fact, you should probably wear clothes you’re not worried about getting damage.

Brush Thoroughly

Brushing your pool after adding shock helps to distribute the chemical while removing a layer of film on your pool surfaces so that it too can be treated. A good brushing accompanied with vacuuming and backwashing should always follow a shock treatment (especially when treating algae).

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