What is TDS in Pool Water?
Understand total dissolved solids, why salt pools read differently and when a high reading needs investigation rather than automatic draining.
Total dissolved solids, or TDS, describes dissolved material in the water. It is different from the visible dust and leaves that a filter collects. A TDS number is useful context, but it cannot identify each substance present or tell you on its own whether the pool is suitable for swimming.
Why salt pools need context
A saltwater pool deliberately contains dissolved salt. Its TDS result will therefore differ from a pool that uses a different treatment system. Compare the result with the pool’s history, source water and equipment requirements rather than applying one internet limit to every pool.
Conductivity based meters estimate dissolved solids using a conversion. Check whether the instrument is displaying TDS, salinity or conductivity and use the correct setting and calibration. A value from one scale should not be copied into another without understanding the instrument.
What a TDS reading cannot replace
- Free chlorine or the relevant sanitiser test.
- pH and total alkalinity.
- Calcium hardness and stabiliser where relevant.
- An independent salt test when a chlorinator warning needs investigation.
- Inspection of clarity, circulation and the pool finish.
Investigate the trend
Record the date, units, device and result, along with recent chemical additions and water replacement. Compare with a sample of the refill water where useful. Evaporation, topping up and deliberate draining affect the water differently, so a history is more informative than one isolated measurement.
Do you need to replace water?
Sometimes a particular measured constituent requires a controlled reduction. Establish what is actually excessive before committing to a water change. A high TDS reading alone does not specify how much to remove, and adding a clarifier does not remove all dissolved material.
Any partial replacement needs a safe water level, suitable discharge and a refill plan. Full draining requires professional assessment of the shell, finish and groundwater conditions. Retest the relevant balance after replacement.
What to send for advice
Include the treatment system, pool volume, complete water report, current symptoms and recent additions. Photograph the meter display so units are clear. See salt testing and safe water lowering before making a large correction.
