The level of calcium found in the water used to fill your spa depends on the area where you live. You will probably know the likely calcium hardness of the local mains water. Some test kits will enable you to determine the calcium hardness levels of the mains water or in the spa by reference to the total hardness bar on the strip. The ideal level depends on the type of spa shell you have:
In acrylic spas calcium hardness should be a minimum of 175mg/l.
In tiled spas the minimum level should be 250mg/l
This should provide sufficient calcium to satisfy the waters calcium demand. Calcium hardness can be increased using Hardness plus. In hard water areas with a high calcium hardness level, Spa Stain and Scale remover should be added as per the instructions on the label to prevent scale formation.