What is Calcium Hardness in pools? Calcium hardness is a concentration of calcium ions which appears in the pool water in the form of calcium carbonate. Calcium is naturally present in water, but when it comes to your swimming pool water, it needs to be maintained and prevented from going either too high or too low by keeping it within the target range of 200-300 ppm. If Calcium Hardness is not kept within range, your pool water can become too aggressive or end up scaling the surface of your swimming pool.

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