Calcium Hardness

Calcium hardness

There are two main reasons why pool owners should care about calcium hardness (CH).

Water Balance: Proper CH helps maintain the overall water balance in your pool. An imbalanced pool can lead to problems like scaling, corrosion, and cloudy water.

Surface Protection: The right CH level prevents the water from becoming aggressive or corrosive. If the water is too soft, it can leach calcium from the pool’s walls and floor, causing damage. If it’s too hard, it can lead to scaling, which can clog filters and affect the pool’s appearance.

Many pool owners are confused by calcium hardness levels. This confusion is found even within the swimming pool industry. They believe that high CH in swimming pools causes calcium formation. Remember, the calcium levels in your pool water do play a part, but it is often high pH that causes calcium formation. For example, we have clients with calcium hardness levels in excess of 1,000ppm and yet have perfectly clean and smooth pool surfaces.   For more information about pH see our page that discusses pH in greater depth.

"Very high or very low calcium hardness in swimming pools will damage the surface over time and make water maintenance more difficult"

Calcium hardness in fibreglass pools

Some pool technicians often disagree about what the calcium hardness levels should be in fibreglass pools. This happens because the various types of software used in water testing suggest different levels. For this reason, we strongly suggest that you call the actual manufacturer of your fibreglass pool (eg. Narellan, Barrier Reef, Compass etc.) to find out what CH levels they suggest for your particular pool.  Remember that in many cases they suggest adding some calcium to the water.  A calcium hardness of zero can be harmful to fibreglass pool surfaces.

Many of these manufacturers each have different calcium hardness requirements. Most importantly, if you have incorrect calcium levels it may have implications for your pool warranty. Remember, maintaining the correct calcium hardness level is very important. Fibreglass is not bulletproof and bad water will damage the surface.


calcium hardness in fibreglasss pool
Calcium formation in fibreglass pool

Calcium can form in fibreglass pools when the total calcium hardness is too high or when the water is out of balance.  Poor water balance along with high chlorine can also lead to bleaching of the surface over time.

Calcium hardness in pebble pools

The main reason that your concrete swimming pool may have high calcium hardness is because of the concrete surface. In this case, a concrete surface will release calcium hydroxide into your pool water over time. This may mean that the calcium hardness levels increase even though no calcium-based products are being added to the pool.  However, this increase in calcium is reduced when water is removed from your pool. For example, splashing out water when swimming, or backwashing the filter all remove water (and calcium) from your pool. Because of this, most pools do not suffer from increasing calcium levels over time.

calcium in swimming pool

High calcium hardness damages pools

It is important that your calcium levels are maintained correctly.  If the calcium levels in your pool water are too high for a long period it will cause calcium to form on your pool surface. This is why water testing is often known as balancing the pool water. Remember, it is not about taking levels to the opposite extreme. Just keep your water in balance. Remember the surface can be damaged if your calcium levels are to low or too high.

Remove calcium from pool water

Here is one of the biggest myths about calcium hardness. Some people believe that topping up you pool with fresh tap or tank water will reduce the calcium level over time. This is not true. Remember, adding water does not lower calcium hardness because as water evaporates, the calcium is left behind in the pool. Importantly, water evaporates but calcium does not! This means that all calcium stays in your pool no matter how many times it is topped up. Also, concrete pool surfaces will also release some calcium into the pool water over time, particularly when the pool surface is new. 

We have found that very few products remove calcium from water permanently.  Most of these liquid-based products ‘suspend’ rather than remove the calcium.  This means that once the chemicals have broken down the calcium is simply released again into the pool water.  So, you will add the liquid, and calcium goes down, then after a few weeks, the calcium level is back to where it was before the liquid was added.  Remember if you are not physically removing the calcium out of the water then you have not removed the calcium.   

Reduce pool water hardness without chemicals

An alternative method that works to reduce calcium hardness is to pump out water using your pool cleaner or vacuum hose during periods of soaking rain when your pool is likely to overflow. This method removes high calcium water from the bottom of the pool while at the same time, the rain refills it with fresh water.  Remember that rainwater has zero calcium. Also, doing this several times a year can have a huge impact on the calcium level in your water, and best of all, it’s free!

Magnesium will show as calcium in a water test

Don’t fall for this rookie mistake!  Magnesium shows up in water tests as calcium. This is because the reagents that measure calcium hardness will also include the magnesium salts that are in your water. This means that your calcium hardness reading will be substantially higher, even though the calcium hardness may be at the right level. In short, if you have a tested calcium hardness level of 800ppm the actual calcium level will be closer to 200ppm. Also, make sure your pool shop actually knows that you are adding magnesium salts to your pool.  Finally, this information is verified on the MagnaPool website which discusses this issue in greater detail.


Calcium hardness in MagnaPool

Your pool shop uses testing software so they know what is needed to balance your pool water. Remember, if these software programs are not designed for MagnaPool then you will have a false calcium reading. Importantly, your actual calcium levels may be correct but an inexperienced technician may suggest otherwise. Because of this, they will suggest that you to reduce the ‘high level’ of calcium in your pool water for no reason.

Measuring Calcium Hardness 

Calcium hardness is an important parameter to monitor in swimming pool water. It refers to the concentration of calcium ions (Ca2+) in the pool water. Maintaining the correct calcium hardness level is crucial for the health of your pool surface and the comfort of swimmers.

Calcium hardness is typically measured in parts per million (ppm). You can use a test kit or test strips specifically designed for pool water testing to determine the calcium hardness level. The ideal range for calcium hardness in a swimming pool is typically between 200 ppm and 400 ppm.  Recent research has indicated that higher calcium hardness levels may be better for pool owners because higher calcium hardness levels can help make the water easier to maintain.  

calcium hardness test

Final thoughts

Remember that maintaining proper calcium hardness is just one aspect of pool water maintenance. You should also regularly test and adjust other water parameters, such as pH, total alkalinity, and chlorine levels, to keep your pool water clean and safe for swimmers.  Often calcium hardness gets the blame for water or surface issues that have nothing to do with the calcium level in your pool.