I had to drain about 80 percent of my pool water for some work to be done. Now, after filling it, I'm struggling with some of the chem readings. In particular is CH which is showing a reading of 80 which equates to needing about 20 pounds of calcium chloride. Is calcium chloride just plain salt? What do I buy? I checked the big box stores, but they don't carry calcium chloride. Any help would be appreciated. Thanks.