Calcium hardness helps keep your water clean. If it’s too low, the water becomes too soft and it can feel slimy.
If it’s too high, the water becomes too hard and will become cloudy. It can also cause scaly deposits to build up on plumbing and filters. Recommended levels are between 150 and 400ppm.
Check the hardness level of your main water supply - for example in Greater London the water tends to be hard, whilst in parts of the UK such as the south-west the water tends to be very soft.
Your choice of disinfectant is affected by the hardness of the water and you might need an acid to adjust it downwards - e.g. when using sodium hypochlorite or calcium hypochlorite, or hardener ad bicarbonate to adjust for acidic disinfectants such as stabilised chlorine.