It's difficult to understand how water can be hard or soft. If you jump from a high place into a body of water, you would report that the water is indeed very hard. The concept of a particular supply of water's degree of hardness has nothing to do with how much it hurts when you smack into it at high speed, however. It refers to the water's mineral balance. Pools and hot tubs are dependent upon proper levels to keep their machinery running efficiently, so filling your hot tub with softened water requires immediate balancing treatments.
Hard and Soft Water
Calcium and magnesium are the minerals that most directly contribute to water hardness. Measurements of these and other minerals in the water supply are noted in parts-per-million, or ppm. Hot tub water that is too soft can be corrosive to metal while water that is too hard can cause damaging scaling. Finding the right balance and maintaining that balance through water testing and treatment is the key to a healthy, long-lasting hot tub.
Filling Your Tub
Since many municipal water supplies tend toward excess hardness, homeowners often add a water softener to the incoming pipe. Water from a home softener is not properly balanced for hot tubs, however, so if you must fill your hot tub with a softened water supply, you'll need to harden it up with treatments before it causes any problems. The ideal calcium level for water in a hot tub is 200 to 400 ppm, but before you add a product like Liquid Calcium to your newly filled tub, you must first get the alkalinity and pH in line.
Striking a Balance
Alkalinity is a measure of all carbonates, including calcium, and it determines the water's ability to maintain a steady pH. Low alkalinity requires the addition of a base, like sodium carbonate, and high alkalinity requires the use of an acid. Fiberglass hot tubs are best balanced at between 170 and 250 ppm alkalinity. Once that level is stable, it is then possible to fine tune the pH levels using an acid or a base until your water is steady at 7.2 to 7.8 pH. Finally, when you've balanced your water's chemistry to the recommended levels, harden your water with a calcium additive from the spa supply store until you hit the recommended window. Hardness and alkalinity levels should be tested weekly.