(also called TDS) A measure of the total amount of dissolved material in the water. It is comprised of the spent or carrier chemicals added every time chemicals are added as well as the hardness, alkalinity, chlorides, sodium, magnesium, calcium, etc. The maximum amount in a pool is 2,500 parts per million (ppm). Maximum for a spa is 1,500 over starting TDS. The only way to effectively lower TDS is to drain part or all of the water and replace it.