TDS Meter: A Guide for Aquarists
A TDS meter measures total dissolved solids in ppm by reading electrical conductivity, used to monitor RO/DI purity and remineralisation.
What it is
A TDS meter is a handheld instrument that measures total dissolved solids, the combined content of all inorganic and organic substances dissolved in water. The reading is given in parts per million (ppm), where one ppm equals about one milligram of dissolved solids per litre of water. It is used to monitor water purity and mineral content.
How it works
A TDS meter measures the electrical conductivity of the water, which rises with the concentration of dissolved ionised solids, then converts that conductivity to a TDS value. Conductivity is read in microsiemens per centimetre, referenced to 25 degrees Celsius, and multiplied by a conversion factor that typically falls between about 0.55 and 0.8. Because different salts contribute to conductivity differently, the conversion gives only roughly ten-percent accuracy, and the same TDS figure can represent slightly different mixes of dissolved substances.
What TDS includes
- Dissolved minerals such as calcium, sodium, and potassium
- Anions such as chloride, nitrate, and phosphate
- Other dissolved ions and small molecules
Monitoring RO/DI water
A TDS meter is most useful for checking reverse-osmosis or deionised (RO/DI) water, which should read close to zero ppm when the filters are working. A rising reading after the RO/DI unit signals exhausted media that needs replacing. For reference, fresh water is generally defined as containing less than 1,000 ppm TDS, and a common secondary drinking-water standard sits at 500 mg/L; aquarium RO/DI output, by contrast, should be far lower, often near zero.
Remineralisation
When near-zero RO/DI water is built back up for an aquarium, a TDS meter tracks how much mineral content has been added. This is common when preparing reef saltwater or soft water remineralised with specific salts for shrimp, where target TDS values guide the dosing. For sensitive shrimp such as Caridina, keepers often use RO water with dedicated buffers or shrimp salts mixed to a target reading, while reef tanks are topped up with RO/DI to avoid shifting the water chemistry.
Limitations
A TDS reading shows the total amount of dissolved solids but does not identify which solids are present, so it cannot replace dedicated GH and KH tests for hardness. Because different substances affect conductivity differently, TDS is best used as a relative indicator alongside specific water tests.