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.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.