Specific Gravity

Definition

Specific Gravity (SG) is the density of saltwater relative to pure water at a fixed temperature. Aquarists use SG as a proxy for salinity in reef + saltwater tanks.

Practical use in aquariums

Reef tanks target 1.025-1.026 SG (corresponds to ~35 ppt salinity). Use a refractometer (NOT a hydrometer - hydrometers drift 1-3% within a year). Calibrate refractometer with 35 ppt calibration solution monthly.

How Specific Gravity fits the bigger picture

Understanding Specific Gravity matters because it's connected to broader husbandry decisions: water chemistry is the foundation of every other aquarium parameter - chemistry mistakes show up months later as livestock loss.

Browse the Fast Aquatics care library for full husbandry tutorials covering Specific Gravity in context.