Magnesium Sulfate (Epsom Salt)
Many grocery store versions of Epsom salt contain perfumes and emollients for bathing in, but this is food-grade Epsom salt for conditioning brewing water. (MgSO4)
Use 1 gram per gallon to adjust Sulfate by 103 ppm and Magnesium by 26 ppm. Also adds 108 ppm to hardness.