Reef Tank Salinity: How to Measure, Maintain, and Match Natural Seawater

The difference between specific gravity and salinity, why refractometers beat hydrometers, and the 35 ppt target.

By Saltwater Aquarium Guide Published May 10, 2026

Handheld refractometer, the tool used to measure aquarium salinity — Photo: CEphoto, Uwe Aranas, CC BY-SA 3.0, via Wikimedia Commons (cropped)

Stable salinity is one of the three pillars of reef chemistry. The target is 35 ppt (1.026 specific gravity) — matching natural seawater — with daily variance under 0.5 ppt.

Refractometer over hydrometer

Cheap swing-arm hydrometers drift up to 0.005 SG, which is enough to stress corals. A $30 refractometer calibrated with 35 ppt solution is the only reliable tool.

Daily stability via ATO

As water evaporates, salinity rises. An auto top-off (ATO) replacing evaporated water with RO/DI freshwater is mandatory for stable salinity in any tank under 100 gallons.

Refractometer over hydrometer

Daily stability via ATO

Milwaukee MA887 Digital Refractometer Review: Lab-Grade Salinity in a Handheld

Salifert Test Kits Review: The Hobbyist Standard for Alkalinity, Calcium, and Magnesium

Red Sea Coral Pro Salt Review: Elevated Calcium and Alkalinity for SPS Reefs