Total Dissolved Solids (TDS) is a measure of everything solid that has dissolved in the water. The main importance of testing total dissolved solids is as a method of determining causes of cloudy water. If the TDS rises over 2,500 ppm total, it could cause cloudy water. A partial drain and refill with water having a lower TDS will help. This is true of spas and pools, except for those pools using salt. Because the salt chlorinator uses salt to help generate a chlorine residual, the total dissolved solids will be naturally high in these pools. Salt pools may have TDS as high as 5,000 ppm with no adverse effects. As a rule, however, the water should be refreshed if the difference between the salt and TDS levels exceeds 2,500 ppm.