Mastering Marine Salinity

    Specific gravity is the heartbeat of a saltwater tank. Learn how to measure it accurately and hold it steady.

    What is Salinity?

    Salinity is the amount of dissolved salt in your water. Hobbyists usually read it as specific gravity (SG) — the density of saltwater compared to pure water.

    Natural ocean water sits near 1.025 SG (≈35 ppt). Because SG changes with temperature, always measure at a consistent temperature and calibrate your instrument.

    Brackish (1.005)Reef (1.025)Dense (1.030)
    Low / Brackish (< 1.020 SG)
    Below natural seawater. Tolerated briefly for some fish-only or hyposalinity treatments, but stressful for invertebrates and corals.
    Best for: Hyposalinity disease treatment (fish-only)
    Reef Sweet Spot (1.024 - 1.026 SG)
    Matches natural ocean water (~35 ppt). The target for reef tanks, corals, and sensitive invertebrates like shrimp and snails.
    Best for: SPS/LPS corals, clownfish, tangs, inverts
    Fish-Only Range (1.020 - 1.025 SG)
    Slightly lower salinity is acceptable for fish-only systems and lowers parasite loads, but is too low for a thriving reef.
    Best for: Damsels, wrasses, fish-only-with-live-rock

    Why Salinity Matters

    Osmoregulation

    Marine life balances internal salts against the surrounding water. The wrong salinity forces fish and inverts to burn energy fighting osmotic pressure.

    Invertebrate Sensitivity

    Corals, shrimp, and snails have little tolerance for swings. Even a 0.002 SG jump during a water change can trigger stress or bleaching.

    Evaporation Drift

    Only water evaporates, so salinity creeps upward daily. Consistent fresh-water top-off is what keeps the number from sliding off target.

    Fish-Only vs. Reef Salinity — What's the Difference?

    Fish-Only (FO / FOWLR)

    Often run slightly lower, around 1.020–1.025 SG. Lower salinity reduces parasite pressure on fish, and without corals to support there's no calcification demand pushing the target higher.

    Reef

    Held at 1.024–1.026 SG to match natural seawater. Corals, clams and sensitive invertebrates need that stability to calcify and osmoregulate — it isn't a preference, it's a requirement.

    Practical Tips for Managing Marine Salinity

    1

    Refractometer Over Hydrometer

    Swing-arm hydrometers drift with temperature and trapped bubbles. A refractometer calibrated with RO/DI (or 35 ppt solution) is far more reliable.

    2

    Top Off for Evaporation

    Salt does not evaporate — water does. As the tank evaporates, salinity rises. Replace evaporated volume with fresh RO/DI water, never saltwater.

    3

    Match When Acclimating

    Acclimate new livestock slowly toward your tank's salinity. A sudden shift in specific gravity is a major osmotic shock, especially for inverts.

    4

    Stability Beats Perfection

    A rock-steady 1.025 is better than bouncing between 1.022 and 1.027. Automate top-off (ATO) to hold salinity constant between water changes.

    5

    Refractometer Calibration

    Zero your refractometer against RO/DI water (1.000 SG, 0 ppt), then check it against a 35 ppt reference solution. Recalibrate every few months — drift is silent and shows up as a wrong reading you'll trust anyway.

    6

    Hyposalinity for Disease Treatment

    Lowering salinity to around 1.009–1.010 SG over several days can treat parasites like marine ich in a fish-only quarantine tank. Do this gradually, and never in a display tank with corals or invertebrates.

    The Golden Rule: Hold It Steady

    Pick a target in the reef range and defend it. Slow, stable salinity beats a "perfect" number that drifts up and down between top-offs and water changes.

    Frequently Asked Questions

    Most reef tanks target a specific gravity of 1.024–1.026 (about 35 ppt), matching natural ocean water. Corals and invertebrates are sensitive to drift, so stability matters more than the exact number.

    Parts-per-thousand (ppt) measures the actual mass of dissolved salts, while specific gravity (SG) measures water density relative to pure water and depends on temperature. Roughly 35 ppt ≈ 1.026 SG at 25°C.

    Evaporation removes pure water and leaves the salt behind, so salinity climbs. Replace evaporated water with fresh RO/DI top-off — ideally with an automatic top-off system — rather than saltwater.

    Fish-only tanks are often run slightly lower (1.020–1.025 SG) to reduce parasite pressure on fish. Reef tanks are held at 1.024–1.026 SG to match natural seawater, since corals and invertebrates need that stability to calcify and thrive.

    Yes — hyposalinity (gradually lowering salinity to around 1.009–1.010 SG) is a recognized treatment for parasites like marine ich in fish-only systems. It's not safe for reef tanks, since most invertebrates and corals can't tolerate the drop.

    Related water chemistry guides

    Salinity is one piece of overall water chemistry — these cover the rest.