Salinity is the foundation every chemistry reading sits on. If your refractometer is off by 2 ppt, every calcium and magnesium reading is skewed too. Target for most reefs: 35 ppt, which is about 1.026 specific gravity at 25 °C (77 °F).
Use 35 ppt fluid, not distilled water
Many optical refractometers are factory-calibrated for brine (NaCl), not seawater. Calibrating to 0 with RO/DI can leave you 1–2 ppt high on seawater. Always calibrate with a 35 ppt seawater calibration standard, then check that a fresh mix of your salt also reads near 35 ppt.
Calibration steps
- Bring the refractometer and calibration fluid to room temperature.
- Place a few drops of 35 ppt fluid on the prism; wait ~30 seconds.
- Adjust the screw until the scale reads 35 ppt (or 1.026 SG).
- Wipe clean, then measure tank water the same way.
- Recalibrate every few weeks, and after any drop or temperature shock.
Digital conductivity meters and quality marine hydrometers are alternatives; they also need periodic checks against a standard. Full background: salinity and RO/DI guide.
Common mistakes
- Calibrating with tap or RO/DI water on a seawater-scale refractometer
- Ignoring temperature compensation (ATC still needs room-temp samples)
- Leaving salt crust on the prism
- Mixing salt to "1.025" without knowing whether that means SG or ppt
Sources
FAQ
Is 1.025 or 1.026 better? Both are fine if stable. 1.026 ≈ 35 ppt at 25 °C is natural seawater.
Can salinity affect alkalinity tests? Extreme salinity error can affect ion concentrations; keep salinity correct first.