Calcium hardness in water

Topping up pool

Facts and Myths about Calcium Hardness in pool water

As with every other aspect of water chemistry, calcium hardness in water 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. Remember, the trick is to maintain the calcium at the right level.


  • 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 novice pool technicians think that a ‘calcium hardness reducer’ will permanently lower the calcium level. However, this is simply not the case.  This is because the calcium isn’t actually removed from the water with these treatments. Remember, these ‘reducers’ work by using chemicals to bind up (or capture) the calcium temporarily.  These treatments do reduce the effects of high calcium in the short term.  However, after 4-6 weeks, these products are completely broken down.  Remember, when this happens, the calcium is simply released from the chemical bond and then returns to the water.  Finally, your calcium level is back to where you started!

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

Remember, simply adding more water does not lower calcium hardness levels. This is because as water evaporates the calcium does not. The calcium left behind will stay in the water. Remember that water evaporates but calcium does not! So, the calcium stays in your pool no matter how many times you top up your pool. Also, concrete pools will also release some calcium into the pool water over time, particularly when the pool surface is new. Remember to check your water levels regularly, especially when the pool is new.

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

The only way to properly lower your calcium hardness level is to pump out (through back washing or draining) a small amount of existing pool water and then refilling This lowers the level because you are taking calcium-rich water out of the pool and replacing it with tap water with very low calcium levels 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 at all So, following this “drain and refill process” will lower the calcium level without chemicals So, if you live in a region that has ‘hard’ water or mineral-rich water, then have the calcium level of your tap water measured before starting this process. Remember, you don’t have to drain the entire pool to lower the calcium level. In many cases, replacing as little as 30-40 cm’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, you can increase it by adding calcium chloride into your pool water. This is very cheap to buy 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 Remember, bringing the calcium to the right level is inexpensive, permanent, and will make the water feel great again.
If you don’t know your calcium hardness level, we suggest getting it tested as soon as you can.  Also, 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 way to reduce calcium hardness is to pump out water using your pool cleaner or vacuum hose. Doing this during long periods of soaking rain when your pool is likely to overflow is best.  In short, this method removes the high calcium water from the pool while at the same time, the rain refills it with fresh water that contains no calcium. So, doing this several times a year can have a huge impact on the calcium levels in your water.  And it’s free!

Calcium hardness in fibreglass pools?

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

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.

Calcium hardness in MagnaPool systems

Inexperienced pool technicians and pool owners don’t understand that Magnapool calcium readings are unique So, unlike a typical salt pool, ‘total calcium’ in MagnaPool includes the ‘magnesium salts’ which run the MagnaPool system.  These salts appear as part of the total calcium test result.  Importantly, for MagnaPool systems, 70% of the ‘total hardness’ reading actually includes the magnesium salts and the calcium Therefore, the tested calcium hardness reading for magnesium pools is substantially higher than typical salt pools Remember to compensate for the magnesium content in the water when testing for calcium For example, if your MagnaPool is running a measured calcium hardness of 200-250ppm, then your actual calcium level is too low and you need to increase it right away MagnaPool recommends a ‘total hardness’ level of around 800ppm compared to 200-300ppm for typical salt pools.

How do I remove calcium 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. Often, some people advise that reducing your calcium hardness will dissolve the calcium on the pool surface. This is not true. For example, maintaining the calcium 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 pool, please contact us for information. Remember, we can help you remove these without draining your pool water.


Suggested calcium levels for MagnaPool

NSW Health

High calcium levels damage fibreglass

Calcium hardness of Sydney tap water