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measurementµS/cm

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.

The freshwater to marine salinity continuum, with aquarium landmarksA horizontal bar from 0 to 40 parts per thousand, divided into the standard salinity classes: fresh below 0.5, oligohaline to 5, mesohaline to 18, polyhaline to 30 and marine above 30. Aquarium landmarks are marked beneath: most so-called brackish aquarium fish are kept between 1 and 15 parts per thousand, a juvenile green spotted puffer sits near the lower end and an adult near full seawater, and the estuarine habitat class as a whole spans 1 to 20.FreshOligohalineMesohalinePolyhalineMarine0510203040Salinity, parts per thousandEstuarine habitat class, 1–20Where most brackish aquarium fish are keptJuvenileAdultSeveral estuarine species move up this scale with age. A green spotted puffer bought as ajuvenile and kept in fresh water is not merely at the wrong salinity — it is at the wrongsalinity for a life stage it left years ago.
Salinity is a continuum and the trade sells it as three categories. 'Brackish' spans a fortyfold range, and a species that needs the top of it will not survive at the bottom. Source: Venice system salinity classes; AquariumHQ habitat class envelopes for the aquarium landmarks.

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.