Reef Utility

Salinity Correction Calculator

Compute how much fresh RO/DI water to swap in to lower your tank's specific gravity (SG). The tool proposes a safe step‑down plan (≤0.002 SG per day). Supports gallons and liters.

Calculator

Freshwater to swap
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Percent of system volume
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Suggested step‑down plan

    Show math

    We assume salinity ∝ (SG − 1). Freshwater swap fraction f is: f = (SG_now − SG_target) / (SG_now − 1.000). Freshwater volume = f × system_volume (remove this amount of saltwater, then add same volume RO/DI).

    Salinity Converter & Calibration

    Converter uses a simple linear approximation around 25 °C / 77 °F: ppt ≈ (SG − 1.000) × 1325. For precision, calibrate with a 35 ppt standard solution (target readout ≈ 1.0264 SG).

    Refractometer calibration quick‑check

    1. Rinse prism with RODI; dry with lint‑free tissue.
    2. Apply a drop of 35 ppt/1.0264 standard solution.
    3. Wait ~45–60s at ~25 °C/77 °F.
    4. Adjust until it reads 35 ppt (or 1.0264 SG).
    5. Rinse & dry; measure tank water and record.