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How to Control Water Hardness in Swimming Pools

By Jack Gerard ; Updated July 21, 2017
Swimming pool water hardness should be monitored to prevent pool damage.
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Maintaining proper water hardness in a swimming pool is an important task. Hard water clogs pipes and pumps with deposits, making the water cloudy and creating a scale on the pool filter. Water that isn't hard enough will damage metal, concrete and tile in the pool by dissolving the minerals they contain. You should test your water regularly with a pool tester that contains hard water test strips to monitor the total hardness of the water. Water hardness below 200 ppm is too soft for pool water. Hardness above 400 ppm is too hard.

High Water Hardness

Drain water from the pool. There aren't any chemicals that can be added to pool water to reduce its hardness, so the only way to lower water hardness is to remove some of the water and replace it with water that is softer. If the water hardness is between 400 and 500 ppm you should drain half of the water from the pool; if it is over 500 ppm you should drain 2/3 or more of the water. If the pool has developed scale deposits due to the high water hardness, drain all of the water and clean the pool liner before refilling.

Test the hardness of the water you'll add to the pool before you start refilling. To reduce the water hardness of the pool water your source needs to have softer water; if it's too high then you won't see a significant reduction. Hook a water softener up to your water source if necessary to reduce the hardness of the water. The average of the hardness levels of your existing pool water and the new water should fall between 200 and 400 ppm.

Refill the pool using the softer water. Allow it to mix and settle for at least 24 hours. Retest the water hardness of the pool once sufficient time has passed. If the pool water still has a hardness over 400 ppm, repeat the draining and refilling and test again after 24 hours.

Low Water Hardness

Determine how much calcium chloride solution needs to be added to your pool water to increase the water hardness and bring it within the optimal hardness range. Consult the packaging or usage instructions to see how much calcium chloride should be added to your pool based on the number of gallons of water your pool holds.

Mix the calcium chloride with water, stirring gently to ensure that it dissolves completely. Pour the calcium chloride solution into your pool water. Once all of the solution has been added to the water, brush the bottom of the pool to agitate the water and to aid in the dissolving of any undissolved particles that settled to the bottom of the pool.

Allow the water to mix and settle for at least 24 hours. Test the hardness of your pool water again after the time has passed. If the water is still too soft, mix and add more calcium chloride to increase the water hardness further. Wait 24 hours before testing again.


About the Author


Born in West Virginia, Jack Gerard now lives in Kentucky. A writer and editor with more than 10 years of experience, he has written both articles and poetry for publication in magazines and online. A former nationally ranked sport fencer, Gerard also spent several years as a fencing coach and trainer.