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Everyone has certainly heard of the term “hard water” and probably either when it comes to drinking water or the need for laundry detergent. But the question is, have you ever taken the two minutes to learn what it means?

For pool owners, this technical term is very important. In fact, water hardness is a fundamental part of pool maintenance. Only if you know what is meant by it, you can avoid sometimes expensive mistakes in pool maintenance.

Maintaining the proper calcium hardness in your pool is essential to reducing the likelihood of corrosion, scale buildup and cloudy water. These are all potential consequences of unbalanced calcium levels in the water.

What is calcium hardness?

Calcium hardness describes the concentration of calcium ions in water. Adding the calcium hardness and the magnesium hardness gives the total water hardness. The ideal range for calcium hardness is 150 to 400 ppm.

Water with a higher calcium hardness is called hard water. Water with a lower calcium hardness is called soft water. Swimming pool water that is too hard or too soft can cause problems in pools.

The concentration of calcium ions in water depends primarily on the filling water. While rainwater is predominantly soft, hard water occurs mainly in regions with a lot of sandstone and limestone. Over time, the calcium hardness of the pool increases due to evaporation of the pool water.

How often should calcium hardness be tested?

Calcium hardness needs to be checked less frequently than pH or alkalinity. The common practice is to test for calcium hardness every two weeks.

It is best to check the water sources from which you take the fill water for the pool. Then you will have some reference points with which to roughly estimate calcium hardness.

You can record calcium hardness with special test strips or pool testers. Usually, both types of testers do not cover calcium hardness. You will have to do some searching at this point.

Pool and Spa Test Strips - Quick
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Some test strips record total water hardness, which is approximately the sum of calcium and magnesium hardness. However, you cannot estimate calcium hardness from the total water hardness value.

What happens if the water is too hard?

If the calcium content is too high, the pool water becomes cloudy. The problem is that even pool shockers can’t get the cloudiness out of the water – no matter how much they use.

In water that is too hard, insoluble calcium soaps and lime deposits also form on the walls and in the pipes of the filtration system.

What happens when the water is too soft?

When the calcium hardness is below 150 ppm, the water can become corrosive. This means that soft water will attack the pool deck and metal.

Grout, concrete and joints get pitted, metal on pool accessories rusts and metal joints on vinyl pools get destroyed. Stains on the walls and pool floor are also possible.

In a pool, this can lead to all sorts of damage that results in expensive repairs. Furthermore, do not use the pool until the calcium hardness is brought back down to a normal level.

Adjusting the calcium hardness

Incorrectly adjusted calcium hardness can cause major problems. Consequently, it is important to balance the concentration properly.

Prepare the adjustment by following these steps:

  1. Clean the pool and remove contaminants.
  2. Adjust the pH and alkalinity before tackling calcium hardness because the other chemicals will affect calcium hardness.
  3. Test the pool water for the concentration of calcium ions.
  4. If the calcium hardness is above 400 ppm, you need to lower the level. If the level is below 150 ppm, you will need to increase it. It is best to aim for a concentration of 275 ppm to be right in the middle of the optimal range.

Increase the calcium hardness

Increasing the water hardness is much easier than lowering the value. You can increase the hardness level by adding calcium hardness enhancers.

Follow the manufacturer’s instructions when adding the agent to get the concentration right.

Clorox Pool&Spa 12204CLX Calcium
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Lowering the calcium hardness

Lowering calcium hardness is a bit more complicated than increasing water hardness. There is not one solution, but different methods how you can help yourself.

  • Partially replacing the water is the best option. Make sure that your fill water is softer.
  • In the long run, make sure you have the right water source. You can read the calcium concentration of the water in your area from various water company maps. If calcium levels are too high, look for a source of water with a calcium hardness of 400 ppm or less.
  • There is no chemical remedy for lowering calcium hardness, but water softener can reduce the effects of excessive calcium. Water softener is a chemical that combines with calcium, making it harmless in a sense. The likelihood and intensity of cloudiness in the water and problems caused by corrosion decrease.
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  • Use the rainwater, because it contains little calcium. If you don’t pull the pool cover onto the pool, the water level will rise several inches during a decent downpour. Use the rainwater to lower the calcium concentration. The best way to get rid of debris that the rain washes into the pool is with shock chlorination.
  • Using a flocculant is another option. When you add such an agent, the excess calcium clumps together. The (small) clumps will accumulate at the bottom of the pool. From there, you can vacuum them up. The filter will also collect the particles. Clean the pool’s filter afterwards.
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Maintaining normal water hardness can be a difficult endeavor. Without a doubt, it is one of the most annoying tasks of a pool owner.

It becomes especially annoying when the water from the main source is too soft or too hard. In this case, your actions are constantly required.

If you live in a region with water that is too hard or if you get your fill water from sources with high calcium content, you should make the use of water softeners part of your regular maintenance schedule.

None of this is difficult. With a little time and patience, you can bring your pool’s calcium levels back to acceptable levels and swim worry-free without having to deal with the problems.

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.