How Often Should I Change my Pool Water?

There are only a few reasons to be changing your pool water, and it is quite a big task. Luckily, most problems with swimming pool water can be fixed chemically or manually cleaning, but there are a few exceptions where you shouldn’t avoid changing your pool water.

You should only have to change your pool water every 5 to 7 years when your Cyanuric Acid (CYA or Stabilizer) levels are too high, above 100ppm. Similarly, you should only have to replace your pool water if you TDS (Total Dissolved Solids) is more than 1,500ppm.

What is TDS (Total Dissolved Solids)

TDS is the measurement of the total amount of solids such as metal, minerals, and salts, that are dissolved in the pool water. The recommended TDS levels in pool water are around 1,200 and the maximum being 1,500 (

Are high TDS Levels bad in pool water?
If you allow your TDS to remain at high levels, staining will develop. High TDS will also diminish the effectiveness of important chemicals in your pool such as chlorine. High TDS will also make the water appear more cloudy.

There is no chemical that can lower the TDS in your pool, so if your TDS exceeds 1,500 you will need to change out your pool water.

What is Cyanuric Acid?

Cyanuric Acid Also Known as CYA, Conditioner or Stabilizer can easily be described as “sunscreen for your chlorine”. Without Cyanuric Acid, direct sunlight can break down the chlorine in a matter of hours. (

Cyanuric Acid protects the free chlorine in your water from being destroyed by the sun’s ultraviolet rays. This reduces the amount of chlorine needed to sanitize your pool’s water, keeping your pool safe and clean.

How to Test Cyanuric Acid Levels

Ideally, your Cyanuric Acid levels should be between 30 and 50 ppm (parts per million) but never more than 100ppm. Indoor pools need very little to none.

To test for Cyanuric Acid, I suggest that you use the Aquachek 7 test strips. You can also take a sample of your pool water to your local pool supply store and they might test it for free.

You only really need to test your cyanuric acid levels at the beginning and at the end of each swimming season as these levels change very slowly.

It is important to test your cyanuric acid levels are high enough at the beginning of the swimming season because you need to make sure that the chlorine is going to be an effective sanitizer in the water.

Why is Low Cyanuric Acid Bad?

Low cyanuric acid levels mean that any chlorine you add to your pool water is going to be vulnerable to damage from the sun’s UV rays.

This means that your chlorine will be destroyed before it gets a chance to disinfect your water and you will have to be constantly adding more chlorine daily, which is impractical, to say the least!

If your cyanuric acid levels are too low, then you are going to have to add a stabilizer, which is more cyanuric acid.

How to Add Cyanuric Acid

You can get cyanuric acid from a local pool supply store. Buy a 5lb’s of cyanuric acid, this should be more than enough. This chemical also goes by the name of CYA, conditioner, or stabilizer. These chemicals are also available online.

Open your skimmer and clean out any debris in there, then pour in NO MORE THAN 1lb per 100,000 gallons of cyanuric acid into the skimmer basket of your pool. Now let your filter run and dissolve the cyanuric acid quickly. After two hours, test the water again using a test strip, or by taking another sample to the pool supply store.

If your levels are now between 30 and 50ppm, you are done. If they are still too low, then you can repeat the process adding only 1lb per 100,000 gallons at a time, until the desired levels are reached.

What happens if I add too much cyanuric acid?
Be careful not to over saturate your water with cyanuric acid because it is irreversible, and you will have to change your water out.

High Cyanuric Acid

So you’ve tested your pool’s cyanuric acid levels and they are nearing 100ppm, too high! What do we do now?

What do high cyanuric acid levels do to your pool?
When there is too much Cyanuric Acid in your water, the efficiency of chlorine is drastically diminished. High levels of cyanuric acid bind to chlorine, slowing its ability to kill bacteria, algae, and germs. This is known as chlorine-lock. High levels of cyanuric acid will also cause your water to appear cloudy.

If the cyanuric acid is too high, then you will have to change your pool water.

Ideally, your cyanuric acid levels should not ever be higher than 100ppm. However, if it is the start of the swimming season, I would suggest postponing a pool change to the end of the swimming season.

What do I do if my cyanuric acid levels are VERY high?
Having said that, if your cyanuric acid levels reach 150ppm or more, then you are going to have to do an immediate change of water because it’s going to be impossible for the chlorine to have any sanitizing effect on your pool water.

What chemicals can I use to lower my cyanuric acid levels?
There are rumors of chemical products that can lower cyanuric acid in the water, but most of them are gimmicks and it is widely accepted that the only true way to fix this, is to change the pool water.

How to Change Your Pool Water

When is the best time to drain and refill your pool?
The worst time to drain your pool is in the summer. The heat can dry out your pool finish and the heat can cause expansion of the structure, causing cracks that are incredibly expensive to repair.

For this reason, if our pool water is due to be changed, we should wait until the end of the summer season to do this.

Should I change the pool water myself?
Changing your pool water is a huge task and I highly recommend you make use of a professional who has insurance in case there is any damage caused as a result of this process.

Preventing High Cyanuric Acid Levels

Avoid chlorine tablets if possible and use granular chlorine instead.

Some chlorine tablets use cyanuric acid binders. This can be the culprit of high Cyanuric Acid in some cases and will decrease the “life span” of your pool water. Look out for “Sodium dichloro-s-triazinetrione” and “Trichloro-s-triazinetrione” in chlorine products and avoid those.


Changing the water of your pool is no small task, so I recommend you only do this when you absolutely need to. Changing the pool water is only necessary every 5 to 7 years under these three circumstances:

  • Your cyanuric acid levels are too high, above 100ppm
  • The TDS of the pools water is above 1,500ppm
  • When you need to repair the pool’s structure or finish

Unbalanced cyanuric acid levels can diminish chlorine’s ability to sanitize your pool water. When it is too low, the sun’s UV rays will destroy the chlorine in your water before it has a chance to do its job. When cyanuric acid levels are too high, the chlorine cannot bind dissolve into the over-saturated water, and again will be unable to effectively sanitize.

You should be testing your water’s TDS and cyanuric acid levels before and after your swimming season.

If your cyanuric acid levels are too low, then you can add some using the process described earlier in this post. Never add more than 1lb per 100,000 gallons at a time because it is impossible to reverse if you add too much.

High TDS levels above 1,500ppm also render the water over-saturated, diminishing the effectiveness of important chemicals. High TDS levels can also cause staining on the surface of your pool.

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