You will use a Total Dissolved Solids (TDS) meter to determine how much calcium and magnesium you are adding to the water. The goal is to increase the TDS level around 25 mg/L above the TDS level in the RO water.How to do this is described below.dding calcium and magnesium to your RO water You need to add the mineral supplement in sufficient quantity to increase the TDS of the RO water by 20 to 30 after 24 hours with the mineral supplement in the water. For example, if your RO water has a TDS of 4 then you want it to reach between 24 and 34.