Pool chlorine tablets - 4 things you must know

Chlorine Tablets

Pool chlorine tablets are quick and easy…but be careful

Some pool maintenance companies continually use chlorine tablets as part of their regular service routine. While this can make life easier for them, the consequences of using chlorine tablets in the long term can actually make the water unsafe. This is because the CYA stops the chlorine from working properly.  This is one reason why the US Government has placed tight restrictions on the use of cyanuric acid (CYA) in public swimming pools. In short, too much CYA locks up your chlorine which renders it nearly useless. This is how your water becomes unsafe even though there may be chlorine in the water.

“Each chlorine tablets contains cyanuric acid. This means that using tablets continually will increase the cyanuric acid in the pool water to unsafe levels. This will reduce the effectiveness and killing power of your chlorine and can make the water unsafe over time.”

#1 – Use liquid chlorine during winter 

It is fine to use chlorine tablets for most of the year.  They will keep the water chlorinated, protected from UV light and also help maintain the water chemistry.  However, we suggest giving the water a break from tablets during winter by using liquid chlorine for a season.  This is because the liquid chlorine will not continually increase the CYA levels in the water like tablets can.  In fact, there is zero CYA in liquid chlorine so using it will mean that your levels will reduce over time.  Also, evaporation and sunlight help break down the CYA in the water. So, using liquid chlorine in winter is the best way to help keep your CYA levels right throughout the year.   

Liquid chlorine drum

Most popular chlorination methods - compared

Liquid Chlorine

Strengths and weaknesses
Liquid chlorine is underrated
  • No electronic equipment needed
  • Cheap to buy
  • Fast acting in water
  • Heavy to transport
  • Will bleach clothes

Chlorine Tablets

Strengths and weaknesses
Chlorine Tablets can make life easy
  • Have built in UV protection
  • Helps keep pH in balance
  • Easy to transport and handle
  • Frequent use blocks chlorine strength
  • Dispenser floating around is annoying

Salt Water Pool

Strengths and weaknesses
Salt is common and easy to run
  • Salt is cheap and lasts a long time
  • Lower maintenance than the others
  • Process is well understood
  • Salt bags are heavy to move
  • contaminated salt causes staining

#2 – High CYA will disable chlorine 

Almost all chlorine tablets contain cyanuric acid (CYA) as well as chlorine. The CYA helps to product the free chlorine from being killed by the UV light from the sun. In theory, this is a great idea. However, with constant use over many months this CYA level will continue to climb. Importantly, once the CYA reaches the point of saturation, it will then impact the effectiveness and killing power of your chlorine.  This can mean that your water will be cloudy and green even though there is plenty of chlorine in the water.  There are several products that can quickly reduce the CYA in your water if this has happened to you.  Or you can just use liquid chlorine until the levels are right again.


When copper sulphate (found in algaecides and pool salt ) interact with high levels of CYA it can leave a stubborn purple stain. This can be repaired but is very tough to remove.  This is why it is important to check  that your CYA levels are maintained.

This client’s pool was always going green even though she was regularly using chlorine tablets. This happened because her CYA levels were so high that the chlorine could not do its job.  In frustration, she added a copper based algaecide.  This turned the pool walls purple because the copper sulphate reacted with the CYA which left a purple stain.

purple stain in swimming pool
Purple staining on fibreglass pool from copper sulphate and CYA

#3 – Chlorine tablets reduce pH 

Chlorine tablets are acidic and as such will help lower the pH in your pool water.  Remember, due to evaporation and sunlight the pH of your pool water will generally increase over time.  This is why your pool shop suggest that acid is needed to balance the water.  However, because chlorine tablets lower pH, it means that your water will not need as much acid to keep the water balanced.  This also means that the amount of acid you will need to add to your pool will be reduced.  This is one reason why chlorine tablets are ideal if you are going away for a period of time.

cyanuric acid

 #4 - Chlorine tablets will keep water clear while your away

Despite all the warnings and considerations, chlorine tablets can be very helpful to pool owners. For example, if your going away for a few months and don’t use an on-site service, then tablets can be very helpful. Place several of them in a floater and leave it to drift around the pool while your away. These floaters will slowly disperse chlorine throughout the pool while your away.  They will also help control the pH. 

However, we do not suggest that you use this method on a permanent basis as your CYA level will get too high.  When this happens, your CYA will reduce the killing power of chlorine and lead to other issues.

Chlorine Tablets

#5 High CYA causes chlorine lock

Chlorine lock happens when a pool’s free chlorine is overpowered by high levels of cyanuric acid (CYA).  When this happens teh CYA binds up the free chlorine levels.  This prevents the chlorine from being able to kill off the algae and bacteria that is now thriving in your pool.  To break this cycle you will need to either chemically reduce the CYA levels or shock chlorinate the pool.  If you have plenty of water you can partially drain the pool as well.  This will reduce the CYA levels.  There is more information about tahe relationship between CYA and chlorine available online.

Mike Johnson
Mike Johnson
Brisbane Australia
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The advice on this website has saved me a lot of time and money.
Mike Johnson
Mike JohnsonBrisbane Australia
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Mike JohnsonBrisbane Australia
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