How to manage Hard Water out of the tap

If your tap water has high calcium hardness or total hardness (over 400 ppm), water balance may be more challenging.

Especially on an initial fill-up (pool startup), it is imperative to test the tap water. All pool chemistry starts at the tap. Factors like calcium hardness can accumulate quickly as water evaporates and leaves. Especially if the tap water that replaces the lost water contains high calcium hardness.

What is Hard Water?

The term "hard water" refers to water with high Total Hardness (TH). TH is the combination of magnesium and calcium hardness. This, of course, is subjective. To some people, anything over 100 ppm of hardness is considered hard. To us, hardness below 200 is low.

In most pools, there is very little (if any) magnesium. Test kits often titrate magnesium out to isolate calcium hardness, which is what the Langelier Saturation Index (LSI) formula calls for. So when people speak about "hard water", they're usually talking about calcium hardness.

Issues with Hard tap water

The main concern with hard water is the precipitation of scale. In swimming pools, the vast majority of scale is going to be calcium carbonate scale, which can be managed and prevented with LSI balance. This occurs when there is an over-saturation of calcium carbonate (CaCO3) in the current chemistry conditions.

There are some other types of scale that high calcium hardness can also contribute to, as calcium ions can bind to other anions like phosphate, silicate, and sulfate, forming:

How to manage hard tap water in a swimming pool

While hard water can be problematic over time, there are options to alleviate it. Before sharing them, consider that most tap water in the United States has below 100 ppm calcium hardness. Municipalities tend to soften the drinking water as part of their treatment processes. 

If your water comes from a public water treatment plant, and you want to know everything in your drinking water, just look up your local treatment facility (probably through your city or county website) and request a full report. They are required by law to disclose everything in the water.

Let's say you're in a desert area like Southern California, Nevada, or Arizona. These dry climates mean faster evaporation rates for swimming pools. The lack of rain means a lack of natural dilution with distilled water (rain). So most of the replacement water will be tap water. These are the pools that struggle with the accumulation of minerals and salts over time. You have options.

Water softener systems

Water softeners are commonly used on homes with hard water. The most common is an ion exchange system that swaps calcium and magnesium ions with sodium ions. The process is described well in this article. A simple rinse with saltwater will release the calcium and magnesium from the filter and recharge the system.

Caution: if your home has a water softening system, that can be detrimental on a new pool startup. If you are building or renovating a swimming pool surface, make sure the pool companies know about the water softener in the house! If you have a hose that is not connected to the softener system, we encourage testing both the hose water and the softened house water. Make the best decision for what fills the pool based on that information.

Reverse Osmosis filtration (RO)

Reverse osmosis (RO) filtration also removes minerals and total hardness. Over time, however, the membranes will need to be replaced and cleaned.

Forced precipitation of calcium carbonate with soda ash

While we do not often recommend this solution, sometimes it is the most economical and practical way to reduce calcium hardness levels in water. The process is explained in detail in this article from onBalance in the Journal of the Swimming Pool and Spa Industry. This process is also known as lime softening. 

Similar to the ion exchange principles in water softeners, the sodium carbonate (Na2CO3), aka soda ash, will interact with the calcium bicarbonate dissolved in the water in the following reaction:

Ca(HCO3)2 + Na2CO3 → CaCO3 + 2NaHCO3

Calcium bicarbonate + Soda Ash → Calcium carbonate + 2 sodium bicarbonate

The article from onBalance (linked above) explains the dosing too:


The potential amount of filterable precipitate that could be generated may be estimated by taking into consideration the following: The molecular weight of sodium carbonate is 105.98 and the molecular weight of calcium carbonate is 100.08. Therefore, one pound of added soda ash reacting with calcium bicarbonate in the pool water can potentially yield 0.944 lbs. of calcium carbonate precipitate. Or, 1.06 pounds of soda ash reacting with calcium bicarbonate can yield up to 1.0 lbs. of calcium carbonate. One pound of calcium carbonate precipitate translates to about 12 ppm of calcium hardness reduction in 10,000 gallons of water.

For easy math, if you want to precipitate and remove 240 ppm of calcium hardness in a 20,000-gallon pool, the dosing math is as follows:

Desired calcium removal = 240 ppm

Pool volume = 20,000 gal

1.06 lbs Soda Ash removes 1 pound of CaCO3, which = 12 ppm CH reduction in 10,000 gallons.

1.06 lbs Soda Ash removes 1 pound of CaCO3, which = 6 ppm CH reduction in 20,000 gallons.


[(desired removal ppm) ÷ (ppm CH reduction per lb. of CaCO3 removed)] = lbs. of CaCO3 precipitation needed

(240 ppm) ÷ (6) = 40 lbs. of CaCO3 precipitation needed


[(40 lbs.) x (1.06 lbs of soda ash)] = lbs. of soda ash needed
42.4 lbs. of soda ash will remove 240 ppm CH in 20,000 gallons of water