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When we talk about Calcium we are talking about a mineral. This is the second type of mineral that you need to be aware of, the first being Alkaline minerals in the TDS section.

The correct level of calcium in your pool will depend on the type of pool surface you have. Vinyl liner, painted and fibreglass pools will need at least 170 ppm of calcium in the water and levels up to 400 ppm are acceptable. Concrete pools will need a higher starting point of at least 225 ppm.

Calcium does several things for you; first it helps the filter be more efficient at doing its job. If your calcium is too low, you may have difficulty getting your pool to clear. Second it relates to the other mineral balance in your pool, Total Alkalinity. As we said in a previous section if the water does not have enough minerals in it, it will look for another supply, such as your liner, so keeping the correct levels of Calcium ensures the water is supplied with the minerals it needs without having to look elsewhere. It is only when Calcium is combined with other minerals, as with those in pool water, it becomes an issue. Temperature is also a factor in overall balance of minerals.

We already know what happens if the water is too low in minerals but what about the other side of the scale when it's too high. What will happen is that as the water has to get rid of some minerals the first thing it releases is calcium, the calcium leaches out of the water and becomes a solid that will deposit on the various surfaces in the pool making them feel rough, like sandpaper. Removing calcium from the pools surfaces is an arduous, unpleasant task. It usually takes about 2 to 4 weeks and requires the addition of a chemical specifically designed to re-dissolve the Calcium back into the pool water and hold it there. It will also need brushing every day, in order to help with the scale removal. 

Note: Low Calcium can cause the water to have a green tinge, not like when you have green algae, it will be perfectly clear but green. When this happens no amount of Shock will fix it, you just need to add Calcium and your water will be back to that lovely clear blue appearance.