Check the listing for details. SpaGuard Calcium Hardness Increaser, Spas and Hot Tubs - 12 oz (3 Pack). Condition: New. Listed at 32.99 USD. SpaGuard's Calcium Hardness Increaser enables spa owners to increase their calcium hardness with this easy to use granular formula, which will boost your calcium hardness to above the danger zone, where low levels cause extensive corrosion of plumbing, jets, fixtures, and tub surfaces. Calcium Hardness Increaser helps raise the calcium level steadily so you don't get too much calcium and start developing scale build-up and cloudy water. The optimal calcium hardness level is 200-400 ppm, while anything below 100 ppm will cause corrosion.SpaGuard's Calcium Hardness Increaser enables spa owners to increase their calcium hardness with this easy to use granular formula, which will boost your calcium hardness to above the danger zone, where low levels cause extensive corrosion of plumbing, jets, fixtures, and tub surfaces. Calcium Hardness Increaser helps raise the calcium level steadily so you don't get too much calcium and start developing scale build-up and cloudy water. The optimal calcium hardness level is 200-400 ppm, while anything below 100 ppm will cause corrosion.