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Pool maintenance is an investment of time and money, among other things, when you need to buy chlorine for your swimming pool and regularly add it to the water in the right dose. Testing the water quality weekly and readjusting it if necessary are also part of a pool owner’s duties.

Only when the values are in the intended ranges can you, the friends, the family or the neighbors safely swim in your pool.

One of the most important values is the chlorine content of the water. Chlorine, a disinfectant, fights pathogens, bacteria and kills algae. The big problem is that solar radiation acts on chlorine or other disinfectants first and then evaporates.

The effect of UV radiation on free chlorine in water is very rapid. According to one study, the amount of free chlorine is halved within 17 minutes.

This lowers the efficiency of the disinfectant, which is actually designed to keep your pool clean, and swimming in the pool becomes a health concern because algae, bacteria and pathogens can multiply in the pool.

The solution to this problem is chlorine stabilizer. This chemical counteracts solar radiation and keeps the chlorine level in the pool constant.

In a pool without chlorine stabilizer, you need to add up to eight times more chlorine than a swimming pool with chlorine stabilizer just to keep the level of chlorine constant. From the first application, you will save money by using chlorine stabilizer. However, there are a few things to keep in mind when using it.

What is chlorine stabilizer?

Chlorine stabilizer is sometimes referred to as pool stabilizer or pool conditioner. The main active ingredient in chlorine stabilizer is cyanuric acid, a chemical that is available in granular or liquid form.

Furthermore, cyanuric acid is also an ingredient in chlorine tablets (trichlor) or pool shockers (dichlor). The terms chlorine stabilizer and cyanuric acid are used synonymously.

Chlorine that is mixed with cyanuric acid is called stabilized chlorine. Unlike non-stabilized chlorine, this is insensitive to the sun’s UV rays.

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If you use one of the products that already contain by stabilizer, normally no additional chlorine stabilizer is required.

On the contrary, if there is too much chlorine stabilizer in the water, there may even be problems. No matter which chemical – there is always too much.

If the chlorine stabilizer reaches too high a value, this can negatively affect the efficiency of the disinfectant. In other words, the stabilizer will do exactly the opposite of what it was designed to do.

It is difficult to reduce the stabilizer content in the water. The only two ways to remove it from the water are either to drain the water (partially) or to add clean water to ensure mixing. With either option, you will also need to rebalance the other chemicals.

How does a chlorine stabilizer work?

Very simply put, the cyanuric acid in the chlorine stabilizer helps the chlorine stay in the water longer and disinfect it. In detail, this means that a chlorine stabilizer binds to free chlorine (chloride ions) and makes it insensitive to solar radiation.

Free chlorine is not stabilized in its ground state and is sensitive to the ultraviolet rays of sunlight. The sun divides the chloride ions, as a result, can evaporate.

With a stabilizer in the water, the chlorine has a disinfection time three to five times longer than it would without a stabilizer.

The big catch with using chlorine stabilizer is that the disinfecting effect of the chlorine (referred to as redox potential or ORP) decreases. As a result, stabilized chlorine (product of chlorine stabilizer and free chlorine) takes longer to kill bacteria, pathogens and algae.

You should never use stabilizer in a spa for this reason. Certain bacteria, for example those that cause folliculitis, are only removed at a high concentration of free chlorine. The slow-acting stabilized chlorine takes 100 times longer to kill these bacteria.

Bathing in such a spa makes your guests and yourself much more susceptible to infection.

In a normal swimming pool, on the other hand, cyanuric acid is a very practical agent that helps you keep the chlorine level constant and save a lot of money in the process. You will have to readjust the chlorine level of the water less often and save money because you will have to buy the chlorine granulate less often.

How much cyanuric acid should you use?

The question of the right amount of chlorine stabilizer is actually quite controversial. Pool specialty stores and internet forums all mention values between 10 and 100 ppm.

The World Health Organization gives 100 ppm as the maximum limit because children who may swallow the pool water can get sick from the high level of cyanuric acid.

We have had the best experience with a concentration of no more than 50 ppm.

The problem with choosing a proper concentration is that a high level of cyanuric acid slows down the action of chlorine, thus favoring a growth and multiplication of bacteria.

On the other hand, a higher level of chlorine stabilizer helps free chlorine to stay in the water longer without being dissolved by UV radiation.

Our guideline value of no more than 50 ppm proves to be ideal, because values above that hardly increase protection from UV radiation, but can increase the risk of bacteria and growth, cause cloudy pool water and other problems with the pool chemistry balance.

A concentration of cyanuric acid below 50 ppm will ensure that the pool still has some amount of free chorine that will properly disinfect the pool. Regardless of the level you set, you will need to perform regular water testing to determine the exact level of free chlorine in your water.

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A healthy balance of cyanuric acid and free chlorine is 14 to 1, which means you should be targeting a cyanuric acid level of 50 ppm and a chlorine level of 3 ppm to 4 ppm.

Reducing cyanuric acid: How to reduce cyanuric acid

In some circumstances, certain bacteria, pathogens and parasites make it necessary to lower the chlorine stabilizer concentration.

For example, for the parasite cryptosporidium, a genus of parasites that enter pools through fecal matter and are highly resistant to chlorine, you should lower the concentration of chlorine stabilizer to no more than 15 ppm.

Unfortunately, it is not possible to lower the level of cyanuric acid by adding a pool chemical. If you want to lower the concentration of cyanuric acid, you must replace some or all of the pool water.

Since cyanuric acid is distributed evenly throughout the pool water, replacing 50% of the pool water will also lower the cyanuric acid concentration by 50%.

We also recommend that you additionally backwash the pool filter.

When and how often to use a chlorine stabilizer?

In pool maintenance, many tasks are scheduled on a daily, weekly or at least monthly basis. In the case of chlorine stabilizer, we are dealing with a very convenient timescale. The chemical only needs to be used once a year. Chlorine stabilizer is neither consumed nor destroyed by the use of other chemicals.

Normally, the concentration hardly decreases over the year. Only in case of large evaporation amounts, a (partial) exchange of the water or dilution by rainwater, you should check the concentration of the chlorine stabilizer and readjust it if necessary.

Chlorine stabilizer is contained in many chlorine products called dichlor or trichlor. Thus, when adjusting chlorine levels weekly, the best case scenario is to add a little chlorine stabilizer to the pool water each time.

If you find any of the following chemicals on the etiquette of a chlorine product, it already contains stabilized chlorine and therefore cyanuric acid:

  • Trichloroisocyanurate
  • Potassium dichloroisocyanurate
  • Sodium dichloroisocyanurate

As a rule, you then do not need to add an additional pool stabilizer. The regular addition of chlorine products with a chlorine stabilizer content is sufficient to prevent the stabilizer content in the water from dropping.

The danger is rather that the pool stabilizer reaches too high a concentration (“overstabilization”). With test strips you can keep an eye on the pool stabilizer level and react if necessary.

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If you notice that the level of stabilizer is moving up week by week, you should first increase the water level to provide dilution.

If the level is still rising, a change to a different pool shocker and to non-stabilized chlorine is in order. With these two changes, you will ensure that the level of stabilizer will no longer increase. Continue to check the water chemistry on a regular basis. Generally, the concentration of chlorine stabilizer should now slowly decrease by dilution.

How to add cyanuric acid to the pool water?

You should never add chlorine stabilizer directly to the pool water, as the acidic pool stabilizer dissolves slowly, it will first settle to the bottom of the pool, attacking it, bleaching the bottom and making it brittle.

You should also refrain from adding the chemicals through the filter. Pool stabilizer has a too low pool pH can damage the filter and other parts of your filtration system.

We advise always dissolving pool stabilizer in a plastic bucket in advance. Do this as follows:

  1. Fill a 20 liter bucket approximately ¾ full with warm water.
  2. The amount of cyanuric acid depends on your pool. As a rule of thumb, you need 0.47 kilograms of cyanuric acid per 10,000 liters of pool water for a 10 ppm higher concentration. You can find more detailed instructions on how to calculate this on the back of the chemical’s package, on the manufacturer’s website or in your pool’s instruction manual.
  3. Now add the correct amount of cyanuric acid.
  4. Depending on the amount of cyanuric acid, you will now need to wait 10 minutes to several hours for the chemical to dissolve.
  5. In the dissolved form, the chemical is harmless to your expensive pool floor and filtration system. You can pour the bucket of water and pool stabilizer solution into the water without hesitation.
  6. Run the pump for several hours to allow the chemical to disperse into the pool.
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Note: Chlorine stabilizer is not used in whirlpools or indoor pools because the free chlorine in the water is also (almost) not exposed to UV rays.

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Pools in outdoor areas should be treated with chlorine stabilizer in any case. Using it correctly will save you time in pool maintenance and money on buying the chemicals.

When using the stabilizer for the first time, you should check the free chlorine and cyanuric acid levels regularly. You may need to do some fine tuning.

Pool maintenance can be a strenuous endeavor that requires regular monitoring. Chlorine stabilizer makes your job easier because you rarely have to readjust chlorine levels, and therefore you can spend more time in the pool. Isn’t that what we’re all about?

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.