Pool Check® TDS is ideally suited for the demanding pool & spa owner who is concerned with proper pool & spa maintenance. TDS, according to the National Spa and Pool Institute (NSPI), is defined as a measure of the total amount of dissolved matter in water, e.g. calcium, magnesium, carbonates, bicarbonates, metallic compounds, etc. When TDS levels in your pool or spa are high, cloudy water, corrosion, and scale can occur. High TDS levels can also reduce the efficiency of chlorine and affect pH balance.