TDS and conductivity
TDS is useful as a change detector — a rising figure between water changes shows accumulation — and useless as a species requirement on its own. Two samples with identical TDS can have completely different calcium, magnesium and carbonate content.
In one sentence
A TDS meter measures electrical conductivity and multiplies it by a conversion factor. It gives a single number for total dissolved ionic content, and says nothing about which ions those are.
Why two meters disagree
TDS is not measured directly. The meter measures conductivity in microsiemens per centimetre and multiplies by a factor its manufacturer chose — commonly 0.5 or 0.7. Two correctly functioning meters can therefore differ by 40 % on the same water. If you compare numbers with another keeper, compare conductivity in µS/cm instead.
Where it genuinely helps
- Shrimp keeping, where remineralised RO water is mixed to a target and consistency between batches matters.
- Detecting a leaching substrate or hardscape early.
- Confirming an RO membrane is still working.