Using HT Liquid Calcium is the easiest way to raise the hardness level in your hot tub. To raise the hardness level 28 parts per million in a 250-gallon spa takes just 2 ounces. Obviously, double that for a 500-gallon hot tub.
This chart shown here and on the bottle provides a simple guide on how many ounces are needed to raise the hardness level based on the number of gallons of water in your spa.
Before adding HT Liquid Calcium make sure your filtration pump is running, then pour it in and wait 30 minutes, then retest and add more if necessary.
Good news is that once you correct your hardness level it usually will stay corrected until you refill your spa.