Calcium hardness measures the amount of dissolved calcium in your pool water. Although calcium doesn't directly affect sanitisation like chlorine or pH, it plays a vital role in protecting your pool surfaces and equipment.

Keeping your calcium hardness within the recommended range helps maintain balanced water chemistry and prevents the water from either becoming aggressive (corrosive) or scale-forming.

Why is calcium hardness important?

Maintaining the correct calcium hardness helps to:

• Protect plaster, concrete and pebbled pool surfaces from erosion.

• Reduce the risk of calcium scale forming on tiles, salt chlorinator cells and pool equipment.

What happens if the calcium hardness is too low?

Water with very little calcium becomes aggressive, meaning it will try to obtain calcium from other sources.

In concrete, pebble and plaster pools, this can cause the water to slowly dissolve calcium from the pool surface, leading to:

• Surface etching and rough finishes.

• Premature wear of plaster or exposed aggregate.

• Reduced lifespan of the pool interior.

Low calcium water can also become more corrosive to some metal components, such as metal ladders, railings, lightings or your salt cell when combined with low pH or low alkalinity.

What happens if the calcium hardness is too high?

When calcium hardness becomes too high, especially alongside a high pH or high total alkalinity, the excess calcium can come out of solution and form calcium scale.

This can result in:

• White deposits on tiles and pool surfaces.

• Scale build-up inside pipework.

• Reduced efficiency of heaters and filters.

• Heavy calcium deposits on salt chlorinator cells, reducing chlorine production and increasing cleaning requirements.

• Cloudy water in some cases.

High calcium levels are difficult to reduce without partially draining and replacing some of the pool water.