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.

Related reef reading

Reef tools mentioned in this article

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Water Parameter Tracker

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