How To Shock A Pool? A Complete Guide

If the water in your pool has lost its crystal clear appearance, if it has changed color, towards green and if you are concerned that it will get worse in the next 24 hours, shock treatment is the best choice.

What Is Pool Shock?

Pool shock is made of a liquid, granular or powdered oxidizer that is called chlorine. Chlorine sanitizes pool water. Shocking a pool means that you add chemicals (normally chlorine) to the pool water to kill the buildup of chloramines, algae, and other harmful bacteria. Large amounts of chlorine need to be added for this process to be effective.

There are 3 different types of pool shock that can be used which are:

Calcium Hypochlorite (cal-hypo) is available in 65% and 73% strength, a pH level of 12, and is not stabilized.

Sodium Dichlor (di-chlor) is stabilized pool shock, with cyanuric acid which is used as protection from the sun, which is available in a 56% strength, with a nearly neutral pH level. It adds no calcium to the pool. If a pool has low levels of cyanuric acid, it is ideal to use a sodium dichlor shock to bring those levels back up. However, too much cyanuric acid can also lead to cloudiness after the shock treatment.

Potassium monopersulfate (non-chlorine/oxidizer) oxidizes pool water in a chlorine-free formula that is not affected by sunlight, leaves nor residue and adds only oxygen.

Cal-hypo (Calcium Hypochlorite) is the best and strongest shock available. Cal-hypo is a quick dissolving, unstabilized pool shock which means that the sun’s UV rays will burn it off quickly and it will not increase the cyanuric acid level in the pool. It is quickly dissolving but must be pre-dissolved before adding it to the pool water. If a pool has high calcium hardness levels you will want to avoid calcium hypochlorite as a shock treatment. Instead, opt for a sodium dichlor or sodium hypochlorite shock treatment to avoid excessive calcium hardness levels. if not you will have cloudy water.

Shocking a saltwater pool is no different than shocking a chlorine pool. Many salt chlorinators have a built-in super chlorinate or shock button. However, using this shock mode of the salt chlorinator only raises the FC level to about 5-7 ppm.

It is also slow, which does not have the desired effect of killing algae, removing chloramines or pathogens. To really kill algae, chloramines, and pathogens you would need to use liquid or granular chlorine.

What Does Pool Shock Do?

Shocking a pool refers to the process whereby adding chlorine to a pool raises the free chlorine level of the pool water in order to kill algae, chloramines, bacteria, and pathogens. This process sanitizes the pool water to make it safe to swim in.

Shocking a pool raises the ph level slightly, and as the chlorine is used up the ph will drop back to normal. The ph level before shock treating a pool should be between the 7.2 and 7.4 range. This ph level is the optimum at which your shock treatment will work at its best. If not at this range you will need to add more chlorine or you can end up with cloudy water.

It is important that the 7.2 to 7.4 ph level is maintained during the shock process for the chlorine to work effectively. The ability of chlorine to work effectively is directly proportional to the ph.

For the best result from shock is to bring the free chlorine level to 40% of the cyanuric acid level in a pool. For example, if you have 50 ppm of cyanuric acid in your pool’s water, add shock until there is a free chlorine concentration of 20 ppm.

When To Shock A Pool?

You might have noticed the instructions on chlorine shocks say they need to be used after the sunset. This is because the sun burn’s off un-stabilized chlorine, which means the shock won’t be as effective. Shocking your pool at night makes sure the chemicals work correctly.

  • These are the conditions where you would need to shock a pool.
  • After the first time, the pool is filled.
  • After emptying and refilling the pool.
  • After winter, when the swimming pool is put back into service.
  • After a thunderstorm, storm, strong winds, hot weather.
  • Following an invasion of algae for whatever reason.
  • After a large number of people swim in the pool.
  • Before the swimming pool is winterized, at the end of the summer season.
  • When a pool is opened in the spring.

What Amount Of Shock Is Added To a Pool?

A simple ratio and a standard rule of thumb to follow when you shock your pool are to dissolve one pound of either calcium hypochlorite or sodium dichlor for every 10,000 gallons of pool water. If you are using sodium hypochlorite, i.e., liquid chlorine, the ratio comes out to 10 ounces for every 10,000 gallons of pool water.

If you are not sure how many gallons are in your pool, here is an easy formula:

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

Another standard measurement point for shocking a pool is to measure the chlorine by parts per million (PPM). To successfully shock a pool, especially one with algae growth, you’ll need to bring the chlorine level to at least 30ppm. 

Another method to calculate how much shock you need is:

  1. To find your Combined Chlorine (CC) reading. To get the CC reading, subtract your Free Chlorine (FC) reading from your Total Chlorine (TC) reading.
  2. Then Multiply your Combined Chlorine CC by 10.
  3. Now Subtract your Free Chlorine FC from that answer.
  4. Using the information on the package, determine the amount of shock that will produce a 1ppm chemical change in 10,000 gallons of water.
  5. Divide your pool volume by 10,000. Take note of this answer.
  6. Next, multiply the chemical change ( step 4) by the divided pool volume (step 5) and the CC/FC difference (step 3).
  7. Convert the answer into pounds by dividing it by 16. This answer tells you how much pool shock you need to add.

You can also use one bag of shock (1 gallon of liquid chlorine) per 10,000 gallons.

How To Shock A Pool?

Prepare the pool and water.

Stop all swimming. explain to the little ones if necessary and close access to the pool if possible.
Uncover the pool. it is probably already uncovered, otherwise, remove the cover or fold up the roller shutter. The shock treatment requires a visual check several times a day.

If the inside of the cover or shutter seems dirty, clean them with a hose. Remove toys, pipes, or anything that was in the pool. Clean and rinse them. Remove any large debris with a net and empty the skimmer baskets and the pump pre-filter.

With a pool brush clean the swimming pool walls, floor, waterline. If the water is really dirty and you cannot see what you are doing, it is wiser to wait until the treatment has started to work.

Remember that once the shock treatment is finished, it will be necessary to clean the walls, floor, waterline, and the submerged parts (skimmers, nozzles, bung, plugs, and various equipment). It’s up to you to assess that situation.

Check the condition of the filter (pressure in particular) and clean it if it seems too dirty. Washing/rinsing of a sand filter, or replacing the cartridge filter.

The most important is the is very important that you check the pH level (it has to be between 7.2 and 7.6), if it is not at an appropriate level, the chlorine will be ineffective and you will have to use an increase or decrease according to the pH level. When the pH levels are correct, then yes, you carry out the shock treatment.

How To Add Chlorine Shock To Pool Water

Fill a bucket with about 15 liters of water from the pool and slowly pour in the granulated chlorine. Attention, always pour the chemical product into the water, never the water in the chemical product. Stir the bucket of water with the chemicals well so that they are dissolved.

Pour the contents of the bucket into a return spout of the pool filter. Do it gently so that it mixes with the pool water and does not settle to the bottom and this way you do not run the risk of splashing yourself and damaging your skin or clothing.

There are always some traces of the granulated chlorine in the bucket, so rinse it with water and pour what is left back into the filter’s return spout. If you want to avoid pouring the chemicals by hand, you can do so with a floating dispenser.

Be patient. Depending on how bad the water was, it can take anywhere from 48 hours to a few days to clear up.

During this time, you must continue to check the water (pH, disinfectant level) and rinse the filter if it starts to build up pressure (rely on the pressure gauge).

When To Add Flocculant to a pool?

If the water has become clear again but remains cloudy, use a flocculant or clarifier to collect the leftover algae to be able to remove them more easily through filtration. It may be the remains of dead algae in suspension. Only use Flocculant if the water has not cleared up.

Once finished, clean the pool: walls, bottom, water line, submerged parts, using brushes, broom-vacuum cleaner, and/or robot cleaner to find clear water.

Recheck and re-clean the filter if necessary before returning to normal filtration mode.

Can You Over Shock A Pool?

You can physically add too much shock to a pool but it’s not a train smash. It will only take longer to clear up the pool water. So the answer is no you can not over shock a pool. Over shocking a pool will do no harm and will remedy its own situation over a longer period of time.

The key here is to first work out how much shock needs to be added according to the size of your pool. When adding pool shock be careful not to test the chlorine levels too soon, as there is a good possibility that the test strips will be bleached which will give a false reading of too little chlorine.

Our normal response would be to add more shock. Then you will have over-shocked the pool. It is best practice to wait a day or two before checking a pool’s chlorine level.

If you have added too much shock then you will have to “wait it out”.

How Long After Shocking A Pool Can You Swim?

The main aim here is to make sure that the pool water is safe enough to swim in. Test the chlorine levels and make sure they are around 5 ppm. Also, it is good practice to wait at least 24 hours after shocking a pool, and only then will it be safe to swim.

Make sure that you can see the bottom of the pool. Check the instructions on the type of pool shock you used, as the instructions will also tell you when it is safe to swim. The right thing to do will be to wait until the pool is clear.

What Happens If You Swim In A Shocked Pool?

If you accidentally or on purpose (I need to swim now scenario) fall into the pool. There is a good chance you will be falling into a problem.

If you swim too soon after a pool has been shocked these following items will cause you problems.

Skin Irritation and Rashes. High concentrations of chlorine will burn your skin. Chlorine dehydrates the skin cells and burns them.
A skin rash will generally also develop from skin burning.
Remedy: Run clean water over your entire body for 10 minutes. For example, shower or garden hose. Call a doctor for advice immediately.

Dryness and Itching of the Eyes. Excessive chlorine drys things out. When you blink is when your eyes get moisture. This stops your eyes from drying out naturally. Swimming in a shocked pool will cause severe itching and dryness of the eyes. Remember you only have two eyes.
Remedy: Hold eye open and rinse slowly and gently with water for 10 minutes. Call a doctor for advice immediately.

Stomach Issues. If you accidentally swallow highly chlorinated pool water you can suffer from vomiting, abdominal pains, aches, diarrhea, nausea, and dizziness.
Remedy: Allow that person to have clean water if they are able to swallow. Call a doctor for advice immediately.

Breathing Problems. Chlorine vapor is extremely toxic and dangerous if inhaled. the vapor affects the inner linings and tubes of the lungs making it difficult to breathe.
Remedy: Move that person to where there is fresh air. If the person is not breathing, begin CPR immediately. Call a doctor for advice immediately.

Discoloration of Clothing. Too much chlorine will fade and even turn your costume white.
Remedy: Rinse clothing for 10 minutes.