7Pros TDS meter is a small hand-held device used to indicate the Total Dissolved Solids in a solution, usually water. Since dissolved ionized solids such as salts and minerals increase the conductivity of a solution, a TDS meter measures the conductivity of the solution and estimates the TDS from that reading. The use of "TDS" (total dissolved solids) meters for analyzing the purity of fresh water has become widespread in recent years. Many aquarists use them to determine if tap water purification systems such as reverse osmosis (RO) or reverse osmosis/deionization (RO/DI) are working properly, or if deionizing resins need to be replaced.