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Facts and Myths about Calcium Hardness

As with every other aspect of water chemistry, calcium hardness also needs to be in balance to protect your pool surface from calcium formation, stains, or corrosion. So unlike pH and chlorine, the calcium level in most pools does not vary greatly over the seasons, so testing every few months is normally sufficient.  Consequently, high or low calcium hardness can cause white staining on pool surfaces, etching, or discolouration. The trick is to maintain the calcium at the right level in between.


  • Pitting on concrete pool surfaces
  • Etching of pool surface
  • Dissolving of tile grout
  • Pitting on concrete pool surrounds


  • Cloudy water
  • Rough surfaces in pool
  • Clogged filters and reduced circulation
  • White ‘blotchy’ areas on pool surface

What about calcium hardness reducers?

Some inexperienced pool technicians believe that adding a ‘calcium hardness reducer’ will permanently lower the calcium level. This is simply not the case because the calcium isn’t actually removed from the water with these treatments. These ‘reducers’ work by using chemicals to bind up (or capture) the calcium temporarily.  While these treatments do reduce the effects of high calcium in the short term, after 4-6 weeks, the ‘reducer’ chemical is completely neutralized and broken down.  When this happens, the calcium is released from the chemical bond and then simply returns to the previous tested level.

Topping up pool
Photo Credit: Truitt's Water Service

Just adding tap or tank water will do the trick, right?

Topping up the pool with fresh tap or tank water will not slowly reduce the calcium level over time. However, simply adding more water does not lower calcium hardness because as water evaporates into the air, the calcium is left behind in the pool. Importantly, remember that water evaporates but calcium does not! Therefore, the calcium stays in your pool no matter how many times it is topped up. Concrete pools will also release some calcium into the pool water over time, particularly when the pool surface is new.

What if my calcium hardness is too low?  Isn’t that good?

The only way to permanently lower your calcium hardness level is to pump out (through backwashing or draining) a small amount of existing pool water and then refilling.  This works because calcium rich water is pumped out of the pool and then replaced with tap or tank water with very low levels of calcium.  Depending on where you live, most tap water in Australia has a calcium hardness of around 50ppm (Sydney water) while tank and rainwater contain no calcium.  Following this “drain and refill process” will lower the calcium hardness level permanently without chemicals!  So if you live in a region that is known for ‘hard’ calcium or mineral rich water, then it may be an idea to have the calcium level of your tap water measured before starting this process. Importantly, you don’t have to drain the entire pool to lower the calcium level. In many cases, replacing as little as 30-40cm’s of water will bring your pool back into the normal calcium range.

How do I adjust my calcium hardness?

If your calcium hardness is too low, it can be easily increased by adding calcium chloride from your pool shop.  Adding calcium to your pool will increase the calcium level right away.  On the other hand, if the calcium hardness is too high, then we suggest pumping out some water from the pool and replacing it with tap water as mentioned above.  Exchanging the water using this method is inexpensive, permanent, and will also make the water feel a lot nicer to swim in.

If you don’t know your calcium hardness level, we suggest getting it tested as soon as you can.  If your pool shop is not equipped to test for calcium hardness, try another shop.  Importantly, all surfaces can react with calcium, the calcium level is important to know (NSW Health).

Calcium reduction…the natural way.

One 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 that has no calcium at all.  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!

Calcium build-up on fibreglass pool
Calcium build-up on fibreglass pool

What about calcium in fibreglass pools?

Some pool shops are unaware that the recommended calcium level in fibreglass pools is often different to concrete pools.  Consequently, some pool shops will incorrectly advise customers to significantly increase the calcium level in their water.  This mistake happens because the water testing software used at pool shops is primarily designed for concrete pools (which are more common) rather than fibreglass.  Most concrete pools have a suggested range of 200-300ppm of calcium while fibreglass pools often have a lower suggested range.

So to ensure that your water is at the right level, check the suggested calcium levels with your pool manufacturer. For example, the pool shown here is from a client who had heavy calcium build-up on their fibreglass pool from poor water balance.  In addition to surface problems, excessive calcium in a fibreglass pool can clog salt cells, damage the heater, and obstruct pipework (Swim University).

Calcium hardness in MagnaPool systems

A common trap for inexperienced pool technicians and owners is understanding that unlike a typical salt pool, ‘total calcium hardness’ in MagnaPool also includes the ‘magnesium salts’ (used to run the MagnaPool system) as part of the total calcium reading. This means that in a pool water test, the high level of magnesium salts in the water will show as calcium.  For MagnaPool systems, 70% of the total hardness reading is actually comprised of magnesium salts rather than calcium.  Therefore, the actual calcium hardness reading for magnesium pools should be substantially higher than typical salt pools to compensate for the magnesium content in the water.  So if your MagnaPool is running a measured calcium hardness of 200-250ppm, then your actual calcium level is too low and should be increased right away.  MagnaPool recommends a ‘total hardness’ level of around 800ppm compared to 200-300ppm for typical salt pools (MagnaPool).

Calcium lumps in swimming pool
Calcium lumps in swimming pool

How do I clear calcium lumps off my pool surface?

If you spot calcium lumps or blotches in your pool, please don’t alter your calcium hardness to try and fix this issue. Some people advise that reducing your calcium hardness will dissolve the calcium formation on the pool surface. This is not true. For example, maintaining the calcium hardness level at either extreme will only make the calcium formation worse, not better. Therefore, the only way to treat calcium formation on a pool surface is to either use a calcium dissolving compound, or to drain and acid wash the pool.
If you have calcium lumps, streaks or blotches forming in your swimming pool, please feel free to contact us for further information. We can help you remove these without draining your pool water.