What Salinity Should a Reef Tank Be?

Most reef keepers hold salinity around 1.025 to 1.026 specific gravity (about 35 ppt). That matches natural seawater closely enough for fish and corals. Measure with a refractometer calibrated per the instructions — hydrometers drift and are harder to read.

Evaporation removes freshwater only, so salinity rises if you forget top-off. Use RO or RO/DI water for top-off, not salt mix. Add fresh RO to the sump or display on a schedule so salinity does not swing daily.

After a water change, mix saltwater to the same target as the display before adding it. Log salinity in ReefDiary after changes and top-offs; slow creep upward is a sign top-off routine slipped.