Maintaining a swimming pool depends heavily on achieving proper water balance. This balance involves managing several chemical parameters, including pH, total alkalinity, and water hardness. When these factors are in equilibrium, the water is comfortable for swimmers and non-destructive to the pool structure. Addressing low water hardness is a common maintenance task, as water that is too “soft” can lead to significant problems.
The Role of Calcium Hardness in Pool Chemistry
Calcium Hardness (CH) measures the dissolved calcium and magnesium minerals present in the pool water. This value is measured in parts per million (ppm) and indicates the water’s saturation level with these minerals. For most pools, the recommended range is between 200 and 400 ppm, though some plaster pools may benefit from slightly higher levels. Specialized pool test kits determine this value, providing the necessary data for adjustments.
When calcium hardness drops below the proper range, the water becomes corrosive, meaning it is “hungry” for calcium. Water naturally seeks equilibrium, and if it lacks dissolved calcium, it will aggressively attempt to absorb it from any available source. This process can cause etching and pitting of plaster, concrete, and grout surfaces by dissolving calcium compounds within the pool’s structure. Low CH also poses a threat to metal components, such as heaters, pumps, and ladders, potentially leading to corrosion and equipment failure.
Maintaining the correct level of calcium prevents this destructive behavior. Keeping the CH within appropriate limits protects the longevity and structural integrity of both the pool shell and its mechanical systems. The CH level helps stabilize the water chemistry, preventing the water from becoming overly aggressive or scale-forming.
Calculating and Applying Calcium Chloride
Increasing the calcium hardness level involves adding Calcium Chloride to the pool water. This product is widely available as a calcium hardness increaser formulated for pool use. The first step is determining the exact amount of chemical required to reach the target range.
Determine Needed Increase
First, calculate the pool volume in gallons and measure the current CH level using a reliable test kit. Approximately 1.25 pounds of calcium chloride will raise the CH level by 10 ppm in 10,000 gallons of water. Using this factor or an online calculator, determine the total dosage needed to bridge the gap between your current reading and the desired level.
Prepare the Chemical
Before adding the chemical, take safety precautions, including wearing gloves and eye protection, as calcium chloride can irritate skin and eyes. It is recommended to pre-dissolve the measured amount of granular calcium chloride in a large bucket of water. Always add the chemical slowly into the water, never the other way around, because the reaction generates heat.
Do not exceed five pounds of calcium chloride dissolved in a single bucket. If the calculated dose is larger, divide the total amount and prepare it in several smaller batches to ensure proper dissolution and safety. Never pour dry calcium chloride directly onto the pool surface, as this can cause localized damage or staining.
Application
With the circulation pump running, slowly pour the dissolved mixture into the deep end or distribute it evenly around the perimeter. The pump must run for several hours—typically six to eight hours—to ensure the chemical is completely dispersed throughout the water. This slow, deliberate application prevents cloudy water and allows the calcium to dissolve fully and integrate into the water chemistry.
Monitoring Hardness and Preventing Scale
After the initial application and circulation, retest the pool water’s calcium hardness level, usually 24 hours later. This retest confirms that the target range has been achieved and that the water is no longer corrosive. If the level is still slightly low, calculate and add a smaller follow-up dose, always following the same careful application procedure.
The risk of over-correction is that too much calcium hardness can lead to the opposite problem: scale formation. Scale is the visible deposit of calcium carbonate minerals that precipitate out of the water. It often appears as crusty, white patches on pool surfaces, tiles, and inside equipment like heaters. This occurs when the water becomes oversaturated with calcium, usually combined with high pH and high water temperature.
Pool professionals use the Langelier Saturation Index (LSI) to determine if the water is corrosive, balanced, or scale-forming. The LSI takes into account five factors to give a single value:
- pH
- Temperature
- Alkalinity
- Total dissolved solids
- Calcium hardness
Maintaining all chemical factors within their ideal ranges ensures the LSI remains balanced, preventing both corrosion from low CH and scale from high CH.