Salinity is a core reef parameter. It affects fish osmoregulation and coral health. Good tools make it easy to stay stable.
This overview compares common salinity testing tools. You will learn what each tool does best. You will also learn how to avoid bad readings.
Hydrometers: cheap, simple, and easy to misuse
Swing-arm and floating hydrometers measure specific gravity. Most hobby models read from 1.020 to 1.030. Reef targets are usually 1.025 to 1.026 at 25°C.
Swing-arm units are fast and low cost. They can trap bubbles on the arm. Bubbles make the reading look higher. Salt creep can also jam the pivot and skew results.
Floating glass hydrometers can be more consistent. They need a tall cylinder and careful handling. They also need temperature correction unless they are ATC rated.
Use hydrometers as a backup tool. Confirm new saltwater with a refractometer. If you only have a hydrometer, focus on repeatability over “perfect” numbers.
- Rinse with RO/DI after each use to prevent salt buildup.
- Tap the body to release bubbles before reading.
- Read at eye level and wait 60 seconds for settling.
For mixing routines, pair this with a stable process. See mixing saltwater the right way for step timing and aeration tips.
Refractometers: the hobby standard when calibrated correctly
Optical refractometers estimate salinity by light bending. Many show both specific gravity and ppt. A reef target is 35 ppt, which equals about 1.0264 SG at 25°C.
Most problems come from calibration. Many hobbyists use RO/DI water at “zero.” That can cause error at seawater levels. Use a 35 ppt calibration fluid for best accuracy.
ATC helps with small temperature changes. It does not fix a hot sample. Let the prism and sample reach room temperature. Wipe with lint-free tissue after each reading.
A real example helps. Your tank reads 1.023 on a refractometer. Corals look dull and LPS recede. You calibrate with 35 ppt fluid and recheck. It now reads 1.026. The issue was tool error, not evaporation.
- Calibrate weekly with 35 ppt fluid, not RO/DI “zero.”
- Use 2–3 drops and close the cover plate fully.
- Rinse the prism with RO/DI and dry after use.
Stable salinity also depends on top-off. Review ATO systems for reef tanks if you see daily swings.
Digital salinity meters and conductivity pens: fast readings with more upkeep
Digital units often measure conductivity and convert to ppt. They are quick and easy to read. Many models resolve 0.1 ppt. Accuracy depends on probe condition and calibration.
Probes drift over time. They also foul from biofilm and calcium deposits. Clean the probe every one to two weeks. Use probe-safe cleaner or diluted vinegar, then rinse well.
Calibration matters here too. Use the correct standard, often 53.0 mS/cm for seawater range. Check your manual. Calibrate at least monthly, or after deep cleaning.
Digital tools shine for water change days. Test new saltwater, then the display. If readings differ by over 1 ppt, slow the change. Aim for under 0.5 ppt difference when adding sensitive SPS.
- Store probes as instructed, often in storage solution.
- Do not let salt dry on the sensor tip.
- Cross-check monthly with a refractometer or known standard.
If salinity is stable but animals still struggle, check other basics. See reef tank water parameters for a full stability checklist.
Sources: Randy Holmes-Farley, “Reef Aquarium Salinity: Measurement and Control” (Reefkeeping Magazine); NOAA seawater conductivity and salinity reference tables; manufacturer manuals for optical refractometers and conductivity meters.
Pick one primary tool and learn it well. Calibrate on a schedule and log results. With stable salinity, your reef will look better and recover faster.
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