Why do we need pool chemicals?

Any pool owner will tell you that pool maintenance requires adjustments to pool chemistry for a number of reason:

• Disease carrying pathogens such as bacteria, viruses and other micro-organisms can grow in un-sanitized water.
• Unbalanced pool chemicals can damage various parts of the pool including the liner and filtration equipment.
• Improperly balanced pool water can irritate the skin, nose  and eyes.
• Water that does not contain the correct pool chemicals can become cloudy and unpleasant smelling

What swimming pool chemicals do you need?

1. A primary sanitizer

To ensure that your pool is hygienic and safe for swimmers you need to introduce a disinfecting agent.  The two main sanitizers used in pools are chlorine and bromine.  There are also saltwater pools but the name is a little misleading as they are not chlorine free.  They simply use a process of in-situ chlorine generation via a process of electrolysis where dissolved salt is used as a store for the chlorine -  instead of adding chlorine directly.

Chlorine is the most popular sanitization method for swimming pools and the product is available in a number of different forms:

• calcium hypochlirite, typically sold in granular form or as tablets
• sodium hypoichlorite, a liquid, often used in pools with surfaces that are sensitive to bleach.
• sodium dichlor, which does not require the addition of cyanuric acid (see section on other chemicals below) and is normally supplied in granular form.
• lithium hypochlorite, usually available as a free-flowing powder.
• Trichlor, mostly sold in tablet form and typically has a high percentage of available chlorine.

Whatever type of chlorine you use, it will react in the water to form hypochorous acid and other chemicals that kill bacteria and other pathogenic organisms through an oxidization reaction.

If bromine is used as the primary sanitizer, it also works through an oxidation process.  Advocates claim that bromine is less harsh and skin and eye irritations are less common that with chlorine.  Adding bromine to the pool is a little more complicated as the product dissolves more slowly than chlorine.  But the process of disinfection is similar.  Bromine reacts with the pool water to form hypobromous acid which acts as the main disinfection agent.   An automatic feeder is generally needed and the overall cost of sanitization can be more expensive.

2) Pool shock treatment

In addition to ongoing sanitization, pool water needs to be “shocked” on a regular basis.  Both chlorine and chlorine free pool shock are available and both involve super-oxidizing the pool water to bring it to a point of breakpoint oxidation (often referred to as breakpoint chlorination in chlorine pools) eradicating contaminants and restoring the disinfectant power of the primary sanitizer.  

3) Stabilizer

In chlorine sanitized pools a stabilization agent is sometimes needed to extend the usefulness of chlorine.  The most commonly used is cyanuric acid and this reacts with the chlorine to form a more stable compound that does not degrade in sunlight.

4) Alkalinity and pH balancing pool chemicals

If the pool water is either too acidic or too alkaline if can cause undesirable side effects.  Acidic water can cause corrosion and damage to metal equipment as well as irritation to swimmers.  Water that is too alkaline can result in water that appears murky and can cause scaling on pool surfaces.  In addition, both chlorine and bromine only work effectively within a narrow band of pH – between 7.2 and 7.8.

The most common chemicals used to reduce high water pH are muriatic acid (ideally in liquid form) and sodium bisufate granules.  But if the total alkalinity of the pool is out of suggested levels, then a pH reducer will also be needed.  If the pH of the pool water is too low this is normally due to low alkalinity which can be adjusted with sodium bicarbonate first before trying to increase the pH.  If the pH does need raising adding a base or alkali should do the trick.

5) Algaecides

Algae can be carried into pool water by the wind and rain.  The three most common types seen in pools are a green floating algae that grows rapidly and can be seen floating on pool surfaces or clinging to the walls, yellow algae which collects as a powdery deposit and black algae which forms in layers and clings strongly to pool surfaces.  Black algae is the hardest to get rid of. Even the best pool vacuum cleaners will struggle to dislodge its long, penetrating roots.

If you keep your pool properly sanitized you should avoid algal blooms. But if they do occur an algaecide may be needed.  The cheapest and most widely used algaecide is quaternary ammonium salts or “quats”.  A stronger form of algaecide is known by the shorthand “polyquats“.  Copper salts and colloidal silver are also sometimes used.

Both chlorine and non chlorine shock treatments area available. With chlorine shock the most common active ingredient is calcium hypochlorite but dichlor and lithium hypochloride are also used. Non-chlorine shock consist of potassium monopersulfate. While both types are usually compatible with chlorine, ozone and bromine treated pools, spas and hot tubs, always ensure you check this on a product by product basis. None of these products can be used in a biguanide pool. Only concentrated hydrogen peroxide can be used as a shock treatment for these pools.

If you have a vinyl liner or delicate surfaces that may be susceptible to bleaching, a non chlorine product may be a better option although some fast dissolving and liquid chlorine shock treatments have been specially designed to avoid bleaching. Non chlorine products are popular for use in spas and hot tubs. In hard water environments (above 200 ppm) a lithium, non chlorine or liquid pool shock will be a better option than calcium hypochlorite products.

You will need to shock the pool weekly or once every two weeks depending on usage.  Determining the quantity of shock treatment to add is dependent on a number of factors:

• The level of combined chlorine in the pool.
• Whether you are using chlorine or non chlorine shock and in what concentrations. For shocking with a chlorine product, you need to add around 10 times the level of combined chlorine. Non chlorine shocks are generally added on a 1 to 1 ratio with each product oxidizing 1 pound of combined chlorine.
• The size of the pool.
• Any specific instructions from the manufacturer. Most products have a pool shock calculator to help with determining dosage amounts.

How To Shock A Pool

• Remove all leaf litter and other debris from the skimmers and filters as these will quickly diminish the pool shock effectiveness.
• Ensure all swimmers are out of the pool. Never allow anyone to enter the pool while chemicals are being added.
• Keep the filtration system running while you shock treat the pool to ensure even distribution with no dead zones.
• Remove all pool accessories, such as floats, from the pool.
• Avoid adding pool shock in windy conditions.
• Try and avoid shock treating during the full sunshine. Many pool users get into a routine of shocking the pool every Sunday evening as this coincides with the end of what is normally the busiest period of pool use.
• Ensure that the product has not expired. The average shelf life for swimming pools shock is around 12 months.
• It is generally a good idea to pre-mix granular shock before treating to ensure that deposits do not remain on the floor. Follow the product instructions.
• Wear gloves when working with pool shock and be careful of splash back when broadcasting over the pool.  Goggles are a good idea.
• Once you have opened a package, do not reseal and reuse.
• Continue to broadcast until you reach a stable level of free chlorine.
• Only allow swimmers back in the pool when chlorine and pool chemistry is in a safe range.

Pool chemistry in brief

When chlorine is added to water as part of the normal pool sanitation regime, it reacts with the water to form hydrochlorous acid. It is this chemical that acts as the primary disinfecting agent in the pool. When chlorine combines with ammonia and nitrates (found in normal pool contaminants such debris, sweat, skin cells, leaves or bird droppings), it forms a combined chlorine compound called chloramine or chloramines. The terms “combined chlorine” and “chloramines” are often used interchangeably.

Still not technical enough for you? Then read more on our breakpoint chlorination page. It provides a more in-depth look at pool water chemistry and chlorination.

It is the build up of chloramines that swimmers will most often notice in a pool, complaining of a “chlorine smell” overhanging the pool surface as well as eye and nose irritation. The smell is a sign that you need to shock treat the pool in order to increase the free chlorine levels (i.e. chlorine that is not combined).

Most experts recommend shocking or super-chlorinating the pool to 10ppm on a regular basis (weekly or every two weeks) during the summer months or when the pool is being used. See more on dosage below.

Choosing the best pool shock for your needs

Chlorine vs non chlorine – The main advantage of using a non chlorine shock (these use potassium monopersulphate rather than chlorine as the oxidizing agent) is that you have less pool down-time. Generally you should be able to use the pool 15-30 minutes after treatment but always follow the specific product instructions. Bear in mind however than while non chlorine shock will oxidize ammonia and nitrates, removing the chloramines, it does not increase the level of chlorine sanitizer in the pool. Chlorine free products are popular for spas, hot tubs, bromine sanitized pools and pools with delicate surfaces that are prone to bleaching.

With chlorine based shock it is vital to ensure that the chlorine returns to safe levels before swimming is resumed. And that takes time. Many pool experts recommend shocking in the evening, running the pump and filter overnight and checking chlorine levels in the morning.

A number of different shock treatments are available with the best pool shock products showcased at the top of this page. When it comes to chlorine shock, the key criteria to look out for are available chlorine levels (the higher the better) and whether the product is quick dissolving.

Calculating pool shock dosage – Calculating shock dosage can be complicated and will depend on (1) the pool shock product you are using (2) the size of your pool, (3) the combined chlorine (chloramines) and free chlorine levels and (4) whether you have specific problems with the pool such as cloudy water or a build up of algae. It follows therefore that the guidance below is only a rule of thumb.

Your starting point is to determine the number of gallons of pool water you have in your pool. If your pool is a rectangular or square shape with a flattish bottom then the calculation will be simpler than if you have a custom shaped pool. But here is a handy calculator to get you on your way.

Shocking a pool can be achieved with any form of chlorine – or by using potassium monopersulphate as the oxidizer – as long as it is added in the correct dosage.

Generally you want to shock the pool to raise the chlorine up to around 10ppm. Remember that shocking a pool is an all or nothing process and it will not work if you do not reach breakpoint chlorination.

Dosage recommendations for calcium hypochlorite, lithium hypochlorite, dichlor and non-chlorine shock products. – These assume that you are carrying out a routine shock procedure and have no extreme conditions such as excessive chloramine build up, cloudy water or presence of algae.  If your combined chlorine levels are high you may need to add more shock treatment so it is always a good idea to test this first with a good quality test kit. You should aim to introduce 10 times the potential of the level of the combined chlorine in order to reach the breakpoint chlorination point.

• If you are using calcium hypochlorite with 65-68% available chlorine then 1lb per 10,000 gallons is a good guide. Dilute it fully first and read the product instructions carefully. If you have delicate pool surfaces take extra care when using calcium hypochlorite as it may cause bleaching.
• If you are using lithium hypochlorite with 35% available chlorine then the basic guideline is to add around 2lbs per 10,000 gallons for a routine shock treatment. This is because the percentage of available chlorine is lower. Lithium is a good alternative to calcium hypochlorite if you are in a hard water environment where the pool is susceptible to scale. As it dissolves quickly it is also a good options for pools with delicate liners. But it is more expensive.
• If you are using liquid chlorine then calculate around 1 gallon per 10,000 gallons for most brands. But read the product details carefully as the dosage amount will depend on the available chlorine percentage.
• If you are using dichlor with 63% available chlorine then 1 lb for 10-12,000 of water is the general guidance for normal conditions. Dichlor contains cyanuric acid which helps stabilize the chlorine and prevent its breakdown in sunlight. This is another good alternative to calcium hypochlorite but is more expensive. It is also likely to result in less initial clouding of the water after immediately application.
• If you are using non-chlorine shock then you are looking at around 1 lb per 10,000 gallons.

• The free chlorine level has bottomed out or the combined chlorine reading is too high.
• The pool water is not completely clear but has taken on a cloudy, dull or murky appearance. Cloudy water is normally a result of dead algae and bacteria rather than pool chemistry.
• Swimmers complain of a “chlorine-like” odor or haze hanging over the surface of the pool. The smell is a result of chloramines (also known as combined chlorine compounds). Chloramines are also the cause of eye irritation.
• There has been any heavy rainfall.  Rain can wash organic contaminants and other debris into the pool.
• There has been an extended period of warm weather and sunshine. High temperature and sun rays speed up the breakdown available chlorine.
• You notice any sign of algae formation.
• There have been periods of heavy bather usage, especially if people have not showered before using the pool.
• There is evidence of a slime on underwater surfaces

When applying a pool shock treatment the goal is to ensure a 1-3 ppm free chlorine (or available chlorine) test reading. You will need to add 10 times the amount of free chlorine as there is combined chlorine in the pool to get rid of. You will need a good quality chlorine kit that is capable of measuring free chlorine levels to ensure that you know that you have reached breakpoint chlorination. You will know you have reached this point when the free chlorine levels start to rise.

When adding the swimming pool shock treatment ensure that the water filtration system is working, that skimmers and filters are clear of any debris and there are no swimmers in the water. If you can, wait until the sun is no longer high in the sky. Broadcast the shock treatment over the deep end of the water and repeat until you reach a stable level of free chlorine.

For more information on pool maintenance, read this article on pool chemicals and our reviews of pool vacuum cleaners.

Shock treating your pool ensures that there is enough freely available chlorine to keep bacteria, algae and pathogens at bay and disinfect any contaminants ensuring the water remains clear and safe for swimmers. Read this article on breakpoint chlorination for a more technical run down on how swimming pool shock treatment works.

As the goal is to get rid of chloramines or combined chlorine in the water, you will need to take a reading of this to determine appropriate dosage levels needed. Test the water using a chlorine DPD kit that can tell the different between free and combined chlorine. The testing process is done in two stages. First a free chlorine reading is taken and then a second reading is taken from the same sample to determine total chlorine.

Combined chlorine is simply total chlorine less free chlorine. The total chlorine level and free chlorine level can be equal at times, but total chlorine is never lower. Once you have your combined chlorine reading you will need to add 10 times the amount of free chlorine in the form of a shock treatment. So if the combined chlorine reading is 1ppm you will need to add 10ppm for treatment.

The size of the pool will influence exactly how many pounds of granular swimming pool chlorine, tablets if using swimming pool chlorine tablets, or gallons, if you are using liquid pool shock, you will need to add to (i.e broadcast over) the water. Follow the manufactures instructions and guidance carefully in making this calculation.

When swimmers complain that they can smell the “chlorine” in the pool, what they are actually referring to is the smell of combined chlorine or chloramines.  Chloramines are formed when chlorine combines with contaminants in the pool water such as sweat, debris, algae, bacteria and skin cells.  Chloramines still have disinfectant properties but perform far less well than freely available (or uncombined chlorine).  The smell is a sign that you need to to “shock the pool” in order to increase the free chlorine levels.

The actual process of getting rid of the choloramines involves reaching a chemical point called “breakpoint chlorination.”  This occurs when enough chlorine is added as part of a pool shock treatment to eliminate the molecular bonds that are combining chlorine to the contaminants.  This will initially have the effect of lowering the chlorine levels in the pool.  More chlorine is added to reach the breakpoint which is when chlorine has reacted with all chemical agents in the water, and then past this point when available levels of chlorine start to rise once more.

Breakpoint chlorination graph

(adapted from a Factsheet by the New South Wales Department of Health in Australia)

This may sound like a complicated process but in practice all you need to do is keep adding chlorine until more than 85% of the total chlorine in the pool is available as free chlorine.  Pool shocking is an all or nothing process.  Not adding enough chlorine to reach breakpoint will result in more chloramines and a lower chlorine residual.  There is no point continuing to shock the pool if you never achieve chlorine breakpoint.  On the other hand, if too much chlorine is added, swimmers should wait until the chlorine residual drops to a safe level before getting in the water.

In working out how much chlorine is needed as part of the shock treatment the first step is to work out the level of the combined chlorine in the pool.  Most tests measure total chlorine and free chlorine with combined chlorine being the difference between these two measures,  Then around 10 times the amount of combined chlorine is needed for a shock treatment. So for example, if combined chlorine is 0.8 ppm, then 8ppm of chlorine is needed to achieve breakpoint.

Read more on pool chemicals.

Pool shock is a form of super-chlorination that has the effect of removing combined chlorine (i.e. chlorine that has combined with contaminants in the water as part of a normal sanitation process) and returning active chlorine levels back to safe levels.

Most pool owners use a granular chlorine shock treatment with calcium hypochlorite as the active ingredient.  The critical data point to look out for when comparing products is the percentage of available chlorine.  A 65% level of calcium hypochorite or available chlorine is the standard.  Any less and the product may not be effective.  The manufacturer may claim that you can swim shortly after using (say) a 30% available shock treatment for example, but your objective of returning your pool to a clean and sanitized condition may not be achieved.  More concentrated versions of pool shock with 75% calcium hypochlorite are also available, often referred to as super plus or super shock treatments. See more on the different types of chlorine in our article on pool chemicals.

But what about liquid pool shock?

Liquid pool shock has pros and cons and your decision will depend according to your specific needs.  As liquid pool shock is quick dissolving and you wont run the risk of damaging (bleaching primarily) the pool liner or any other surfaces.  If you have a vinyl swimming pool, liquid pool shock may be the safer option and is sometimes referred to as a vinyl pool shock.  Remember though that there are some granular shock treatments that are safe to use in vinyl pools.  When it comes to cost, granular shock tends to be a more cost-effective choice.  When considering which product to purchase, do a side by side comparison comparing the available chlorine levels.

We have seen some liquid pool chlorine shock treatments with as little as 10% available chlorine.  Our advice: steer clear of these, however easy they may be to add to the pool.

If you are in the market for non-chlorine shock then consider these two options below. Both are best-sellers offering a highly effective shock treatment:

In The Swim Chlorine-Free Pool Shock – 6 x 1 lb. bags

Click to View Product Details

$21.99

Leisure Time RENU2 Renew Non-Chorine Shock, 2.2-Pound

Click to View Product Details

$12.05

Some of the advantages of non-chlorine shock are listed below:

• Pool downtime is minimized. The best non chlorine pool shock products are quick dissolving, work rapidly and do not need to dissipate. With most products, swimmers can re-enter the pool after only a few minutes have past. This may vary depending on the product used so always follow the specific product instructions.
• The risk of byproducts forming such as calcium or cyanuric acid is reduced.
• Non chlorine shock is safe for use as a spa shock as will not damage delicate spa surfaces. It is also safe as a vinyl pool shock as the product is non bleaching.
• Less affected by heat and sunlight than chlorine products.
• The dosage calculation is simpler than with chlorine shock. If non-chlorine shock is used in excess it will convert ordinary salts present in the pool into free chlorine.
• Many of the best products on the market (including the two posted above) are pH buffered so won’t cause pH fluctuations in your pool or spa.

On the downside, non chlorine shock is generally more expensive than chlorine based shock treatments. In addition as it is not a sanitizer, some pool experts recommend using chlorine based products for pool start ups, and after fresh water spa fills to ensure that enough residual chlorine is available in the water.

Pool shock chlorine

A chlorine pool shock works to increase the levels of free chlorine back to safe levels. Shocking a pool is effectively adding a super dose of chlorine to bring the pool to a chemical point known as breakpoint chlorination where there is enough chlorine in the water to eliminate the bonds that are combining chlorine to contaminants. This process is best visualized by referring to the graph on our breakpoint chlorination page. Chlorine shock is compatible with chlorine, bromine and ozone sanitizing systems.

There are a wide range of chlorine based pool shock treatments on the market. The active ingredients typically used are: Calcium Hypochlorite, Dichlor, Sodium Dichor, or Lithium hypochlorite (also known as Lithium Shock). See more on the different types of chlorine in our article on pool chemicals. The key criteria to consider when comparing different products is the level of available chlorine. Most of the top brands offer shock treatments with 45-75% available chlorine. 65% is recommended by most pool experts with 75% treatments – often referred to as “super shock” or “shock plus” – useful for messy Spring start ups or to rid the pool of stubborn algae blooms. If a chlorine shock is advertised as a blended product, perhaps with water balances, flocculants or clarifies, it’s always a good idea to check for available chlorine levels.

Both liquid and granular chlorine shock treatments are available and you’ll find more information on our liquid pool shock page. Liquid shock tends to be more expensive when available chlorine levels are taken into account but has the benefit of eliminating the potential for damage to pool surfaces. But you can normally avoid bleaching risk by choosing a quick dissolving granular product as this minimizes contact time with the liner and other surfaces. Look our for chlorine shock treatment products that are made with super fine chlorine granules.

While more expensive than chlorine shock treatments, most spa experts recommend the use of a non-chlorine shock containing MPS. This type of shock treatment is oxygen based and is often referred to as OXY shock. It works for pools as well as spas with the advantage that there is less downtime. Swimmers and spa users can usually enter the water soon after applications. (Note: always follow product instructions carefully as some may require a longer wait period.) A second advantage of using non chlorine products for both spa shock and pool shock treatments is that they won’t bleach of harm vinyl liners or surfaces.

Non chlorine spa shock is not strictly speaking a sanitizer so needs to be used with a sanitizer system (chlorine, bromine, ozone etc). For fresh water fills a spa chlorine shock is normally recommended. This can be followed by regular non-chlorine shock treatments after a residual chlorine or bromine level has been established in the spa.

Some non chlorine shock treatments are a blend oxidizer and flocculant combination (sometimes referred to as oxidizing flocculant) with some spa and pool users reporting excellent performance. Chlorine free spa shock is fastier dissolving than chlorine shock and owners do not run the risk of raising water calcium levels. The amount of spa shock treatment to use will depend on the bather load and type of sanitation system being used.

Note: Spa shock will destroy biguanide polymers and as such is not compatible with biguanide pool and spa systems.