Conductivity (often EC) measures how well the water conducts electricity. The current is carried by dissolved ions. Conductivity is therefore usually the fastest way to estimate total dissolved salts — without saying which ions are present.
It is an overview parameter, a bit like temperature for metabolism: one number that quickly shows whether the ionic environment is low, typical or unusually high. For which ions are present you need hardness, alkalinity, individual analyses or salinity. See mineral and ion management.
Units
Common units are µS/cm (microsiemens per centimetre) in freshwater and mS/cm in brackish water and the sea (1 mS/cm = 1 000 µS/cm). Some instruments show TDS (total dissolved solids) in mg/L — that is usually a conversion from conductivity, not a separate measurement. Temperature affects the reading; use temperature-compensated conductivity if the instrument has it.
What low and high conductivity mean
Low conductivity usually indicates:
- Very soft water
- Low mineral content
- Possible need to remineralise (rain, RO, mountain water)
High conductivity may indicate:
- Elevated salinity
- High dissolved solids from the source or from feed/chemicals
- Concentration by evaporation, especially in RAS
Approximate orders of magnitude
| Water | Typical conductivity |
|---|---|
| RO / distillate | very low (tens of µS/cm or less) |
| Soft freshwater | low, often below a few hundred µS/cm |
| Typical freshwater / many farms | a few hundred to about 1 000 µS/cm, depending on geology |
| Brackish water | the mS/cm range, between fresh and sea |
| Seawater | around 50 mS/cm |
Orders of magnitude, not operating targets. The same conductivity can come from different ions.
What conductivity does not say
Two waters with the same EC can behave differently. One may be soft with a lot of sodium chloride, the other hard with calcium and bicarbonate. Fe and Mn barely show in conductivity but can be the largest pretreatment job. EC is therefore complemented by hardness, alkalinity, salinity and — when needed — individual ions.
Use on the farm
- Hatchery and RAS: daily or continuous overview of the ionic environment.
- Source change or make-up: see whether new water is softer or saltier than the system.
- Evaporation in RAS: a slow rise can mean salts are concentrating.
- Remineralisation: EC rises when you add minerals — but also measure hardness and alkalinity.
Differences between systems
- Ponds and flow-through: EC reflects the source and the catchment.
- RAS: EC can drift with feed, chemicals, evaporation and water exchange.
- Marine: EC is high and stable in open sea; in closed marine RAS it can still change.
Practical rule of thumb
Conductivity counts the ions. It does not name them. A sudden change is often more important than the level itself — something has happened with source, salt, evaporation or dosing. See salinity, hardness and mineral and ion management.
Sources/overview (selection): Timmons & Ebeling; Boyd & Tucker; established hatchery and RAS practice. Ranges are orders of magnitude, not permit values.