A high level of calcium can make hot tub water feel "hard." The large amount of calcium also causes stains on a hot tub's walls and makes the water cloudy. All of that makes the hot tub far less pleasurable for you and your guests to use. Reducing the water's hardness requires lowering its calcium level. You can do that with just a few supplies, and you do not have to hire a pool or spa technician.
Open a calcium test kit, and take out its calcium test strips, color chart and manufacturer's instruction manual.
Submerge one calcium test strip in the hot tub water for as long as directed by the test kit's manufacturer. The required time varies among manufacturers, but it is usually only a few seconds.
Remove the calcium test strip from the water, and hold it next to the color chart to determine the calcium level of the hot tub's water.
Pour 1 1/2 oz. of trisodium phosphate (TSP) into a measuring cup for every 1,000 gallons of water the hot tub holds. For instance, if your hot tub holds 500 gallons of water, then pour 3/4 oz. of TSP into the measuring cup.
Pour the TSP from the measuring cup into the hot tub. Let it absorb into the water for one or two hours, and then test the water for its calcium level again. The calcium level should be lower for the second test.