Today I was approached by a client that was having a new Soft Water System added to their home.  The Soft Water Sales Person suggested they purchase the higher-end system to accommodate filling their 35,000 gallon pool for a mere $3000 additional dollars.  It was implied that the use of soft water would eliminate the chlorine smell and scale on surrounding tile.    I should point out they presently have a “chlorine” pool with no salt chlorination system in place today.

After researching several forums on the subject it appears this seems to be a question that may require some clarity.   The following is an excerpt from “Swimming Pool Help Forums”.

A question was submitted by Dan R. with the following post:

My question is this: I have always gone by, if the pool is filled with soft water, you are begging for trouble. I understand water is “thirsty” and will seek things out to balance itself, like for instance, lack of calcium in the water, it will find it elsewhere, like from the plaster? Thus causing damage? 
So, if the above is accurate, why not can one fill their pool with soft water in the event that the pool is on a salt/chlorine generating system. Like home water softeners, the calcium molecule is replaced by the sodium or sometimes potassium molecule, so would that make the water not “thirsty” anymore thus resulting in a lack of damage to the plaster? Seems to me logical, but I am no water balance scientist! 

Here is the response from the forum moderator Larry:

(1)  Your assumptions are absolutely correct. The reason we emphasise calcium hardness in swimming pool water is that naturally occurring hardness in fresh water is mostly a result of calcium. Also the nature of pool construction means the water is able to leach the calcium from the calcium carbonate found in concrete, plaster and tile grout.

The hardness level of water is made up of a variety of minerals, including magnesium, potassium and of course sodium. In fact the most common complaint of pool owners using salt water chlorine generators is scale.

Soft water can be conditioned using salt if you install a chlorine generator, but for fresh water pools adding calcium in the form of calcium chloride is the easiest, cheapest and quickest way.

So in conclusion, it is not important which minerals are responsible for the water hardness level, but it is actually the level of the total hardness that is of prime importance in pool water balance.

(2)  Well Dan, as you have implied, using a softener doesn’t reduce the hardness levels in salt water pools by the very nature of their working principle.

An ion exchange mineral, typically a resin, adsorbs calcium and releases sodium (which comes from the salt brine regeneration). The resin has a greater affinity for sodium so will only absorb calcium when there is no sodium present. But the pool water is salty, so the softener will never “use up” the sodium and get round to dealing with the calcium. Any calcium that is trapped will be displaced by the next sodium ion to happen by.

Hard water is typically treated with a chelating agent to render the calcium inactive. It actually bonds with the calcium ion, keeping it in solution and prevents it from plating out as scale.

Using a water softener on the make up water line will reduce the risk of hard pool water. Close monitoring of the pool water hardness to make sure the hardness is not too low must still be done.

One of the most common sources of hardness in indoor pools not exposed to sunlight is the calcium hypochlorite (non-stabilized “shock” chlorine). If the pool uses cal hypo, regular backwashing is necessary. In the case of a salt water chlorine generator, the hardness will be a result of the salt and the fill water as you mentioned.

The Langelier Index was developed before the advent of salt water systems. At that time hardness was most easily and economically measured as calcium hardness. Nowadays test kits for total hardness are readily available and most test strips show total hardness only.  The Langelier calculations can effectively be done using the total hardness value. I remember reading something about the correlation between calcium hardness and total hardness in the Langelier Index. I’ll try to find it and post the details here.

(3)  Total Hardness vs. Calcium Hardness

Total Hardness = Calcium Hardness x 1.18 

Looking through the reference books again I see the calcium is “required” chemically to create calcium carbonate (CaCO3) to ensure saturation. If we remove all the calcium in a sodium-rich environment, it would make sense that we may end up with an abundance of sodium carbonate (pH up) or sodium bicarbonate (alkalinity up). 

So by removing the calcium that “stabilizes” the carbonate ions, we would end up with water having high pH, alkalinity or both. Perhaps a water chemistry specialist could comment on this. 

It is the calcium carbonate that causes scale in hard water, but it seems we still need a certain concentration in the pool.

Red Square Pools serving S. Nevada, Las Vegas, Henderson & Summerlin.  Services include weekly cleaning, repair, equipment sales, acid wash and tile cleaning.


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Thank you for checking out our blog...we appreciate you taking the time to read our posts. To be quite honest, our posts come from your questions. Red Square Pool's wants to clear up the confusion and frustration of maintaining your pool and spa. We simply want to make Pool Maintenance and Repair SIMPLIFIED!! Let us know what your thinking by dropping us an email at Have a safe and happy swimming experience!

One response »

  1. Maria says:

    It is a tricky subject indeed. Thank you for trying to clarify this.

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