How to lower swimming pool calcium hardness
Is your swimming pool water too hard?  If you have too much calcium and magnesium dissolved in your water, then water is considered hard water.  In this article, I explain how to decrease the hardness level in your swimming pool, and include a few tips on monitoring your calcium hardness level.

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Calcium hardness represents the level of calcium dissolved in your swimming pool water.  If you need a little background information on calcium hardness, I explain more about it in another article: Maintaining Pool Calcium Hardness.  If you want to know how to test, read Testing Pool Calcium Hardness, and learn how to raise pool calcium hardness if its low.

Importance for your pool

Why lower the pool water calcium hardness level?

  • Cloudy water and water scale.  The higher the calcium hardness level, the harder the water.  This means higher levels of minerals dissolved in your water.  Too hard, and the pool water will begin to deposit water scale, leaving behind mineral deposits on pool walls, equipment like filters and the inside of pipes, along with cloudy pool water from minerals that are no longer dissolved in the water and are free-floating.
  • Unbalanced pool water.  Keeping calcium hardness in balance is one of three important water balancing steps.  The other two are pH and total alkalinity.  You should always make sure you have first balanced total alkalinity, then pH, and lastly calcium hardness when balancing your pool.  You ideally want to keep the calcium hardness level balanced, as too low or too high can cause problems for you pool.  The ideal range for calcium hardness in swimming pools is 150-400 ppm (parts per million).  If you keep your calcium hardness in the midpoint between those two limits, about 275ppm, then you should be fine.
  • Reduced pool chemical effectiveness.  Other chemicals intended to adjust total alkalinity and pH will be less effective, and the result is you end up adding more to achieve the same result.

I discuss in more detail problems you can encounter with improperly balanced pool water calcium hardness, in my article: Maintaining Pool Calcium Hardness.

Why is my pool calcium hardness too high?

You may have added too much calcium chloride when raising the pool calcium hardness level.

You might live in an area where your local water source is naturally hard, as this can vary greatly depending on where you live.  You can get more information from your local pool service provider or retailer, as they will have discussed this many times before with other customers.  You can also call your local municipal water or “utilities” company, as they will have information on the hardness level in your area.  If your local water is always hard, then your pool water hardness level will naturally drift higher over time.

Shocking the pool water adds calcium, as the active ingredient pool shock is Calcium Hypochlorite.  Chlorine from the shock treatment is used up, leaving the calcium in the pool water.

How to Lower Swimming Pool Calcium Hardness

There are a few ways you can approach a swimming pool with a hard water (excess calcium hardness) problem: replacing pool hard water with fresh water, using a substance called a flocculant to “clump-together” excess minerals, and by adjusting the saturation index.

Lowering your calcium harness by adding fresh water:

filling swimming pool with water

Refilling a portion of your swimming pool can help bring down the calcium hardness level.

The safest and most reliable way to reduce the calcium of your pool is to remove and add fresh water back into your pool.  If you have a pool pump and filter that has a backwash option, you can use the occasional backwashing you have to to anyway as an opportunity to remove a little extra pool water and refill the pool with fresh water.  Here are a few general instructions on backwashing and refilling your pool:

  1. First test your pool calcium hardness level.  You need to first know how your current calcium hardness level.
  2. Determine how much water you need to remove from your pool.  If you are over the ideal calcium hardness range (400 ppm), you may need to drain about one-fourth of your pool to bring the level down.  This can be an expensive undertaking if your local water rates are high, so you should talk to a local pool service provider first to see what is typically done in your area.  They have experience with this and can give a rough estimation of how much water to drain to avoid unnecessary draining, but make sure you drain enough to affect the calcium hardness level.
  3. Drain a portion of your swimming pool water, either by backwashing or siphoning the water.  You can siphon water out of your pool if your pool is above ground or the ground behind the pool slopes enough away.  Be sure to turn off your pool pump if you siphon the water and expect the pool water level to go below the pump water inlet.  You do not want the pump motor to be running without any water flowing through it.
  4. Refill the water you have drained.  Wait at least six hours for the fresh and existing water to mix together, as the cold source water might have sunk to the bottom of the pool if your existing pool water is warm.
  5. Retest your pool water for calcium hardness and determine if you are within the ideal range, or if you need to lower the calcium hardness level again.

Lowering your calcium hardness with a flocculant:

There are additives called flocculants, that can help attract minerals that have gone out of solution, where your water is very hard (up to and over 400 ppm).  There are a few products designed for pool use, but the success of the product depends on specific water conditions of your swimming pool.  Calcium that is dissolved in the water, or in-solution, likely will not be affected by the flocculant.  These products will really only cause solids already out-of-solution to stick together, so that they can be vacuumed or filtered out of the pool water.  In this case, you may can:

  1. Use a flocculant to rid the pool of free-floating calcium carbonate.
  2. Vacuum the walls and floors of the pool, so the excess calcium carbonate particles are pulled into the pool filter.
  3. Backwash or clean out your pump filter.
  4. Drain a portion of your pool water and refill with fresh water.
  5. Test the pool water for calcium hardness.  With the excess calcium carbonate removed and fresh water added, you can should notice a reduced calcium hardness level.

Using an acid to adjust your pool water saturation index:

You can also add an acid, such as muriatic acid or dry acid (sodium bisulfate).  Adding an acid will not lower your calcium hardness, but it can bring your pool into balance according to the saturation index.  You should ask your pool service provider about this and get their opinion and advice before attempting this, as your pH and total alkalinity will be reduced as well and can cause your pool water to become unbalanced and corrosive.


Keeping the calcium hardness level from creeping too high can prevent most water scaling on pool walls and equipment, avoid cloudy and uninviting pool water, and help to keep your pool water balanced.  Testing and maintaining the calcium hardness level will go a long way to provide a balanced and healthy swimming pool.


  • “Pool & Spa Water Chemistry, A Testing and Treatment Guide, Waterproof Edition, 2005. Taylor”

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We're almost done making an eBook for pool owners like you that want an easy and straightforward guide to keep their pool water balanced and clear with as little work as possible.

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Poolforthought ebook swimming pool guide clean balanced water