Stable salinity keeps fish calm and corals growing. Most problems start with bad measurements, not bad salt. This guide shows how to calibrate tools and confirm results.
Pick the right target and reference
For most reef tanks, target 35 ppt or 1.0264 specific gravity. Measure at 25°C or 77°F when possible. Temperature shifts can skew readings. Always note the temperature during tests.
Calibrate with a proper standard, not RO water. RO water only checks “zero,” not seawater range. Use a 35 ppt calibration solution for refractometers. For conductivity probes, use 53.0 mS/cm at 25°C.
Store standards tightly sealed. Evaporation raises concentration and ruins accuracy. Replace bottles every 6–12 months. Label the open date on the cap.
Decide on one unit for your log. Ppt is simplest for reefs. If you use specific gravity, stick to one temperature scale. Add salinity checks to your reef tank maintenance schedule.
- Reef target: 35 ppt (1.0264 SG at 25°C)
- Fish-only target: 32–35 ppt, based on stock
- Calibration: 35 ppt solution or 53.0 mS/cm standard
Calibrate refractometers the right way
Clean the prism first. Use RO water, then wipe with lint-free tissue. Salt film causes high readings. Let the refractometer sit near the tank for 10 minutes.
Add 2–3 drops of 35 ppt standard to the prism. Close the cover plate and wait 30–60 seconds. This allows temperature equalization. Look through the eyepiece under bright light.
Adjust the calibration screw until the line hits 35 ppt. If your scale shows SG, set it to 1.0264 at 25°C. Rinse the prism and repeat once. Two matching results confirm the set point.
Now test tank water the same way. Take two readings from separate samples. If they differ by more than 0.5 ppt, clean again. Also check for microbubbles on the prism.
- Wait 30–60 seconds before reading
- Calibrate monthly, or after drops and bumps
- Recheck after big salinity corrections
Calibrate digital probes and verify with cross-checks
Conductivity probes need clean electrodes. Rinse with RO water, then soak in probe storage solution. If you see deposits, soak in 1:10 vinegar water for 10 minutes. Rinse well after soaking.
Calibrate using 53.0 mS/cm at 25°C, unless your controller specifies another value. Stir the standard gently during calibration. Do not touch the probe tip. Keep the probe fully submerged and bubble-free.
Verify results with a second method. Compare probe salinity to a calibrated refractometer. Aim for agreement within 0.5 ppt. If they disagree, trust the tool that matches the standard best.
Common mistakes cause slow drift. Air in the probe chamber reads low. Salt creep on connectors causes noise. Add a quick salinity check to your ATO setup guide routine, since ATO issues change salinity fast.
- Probe reads low: check bubbles, flow, and placement
- Probe reads high: clean deposits and recalibrate
- Numbers swing: inspect cable, connector, and grounding
If salinity is off, correct it slowly. Change no more than 1 ppt per day for reefs. Use pre-mixed saltwater to raise salinity. Use RO water to lower it. Confirm with a second reading before dosing anything.
Log every calibration and reading. Include temperature and tool used. Trends reveal evaporation and ATO failures early. For more stability tips, review reef parameter stability basics.
Sources: Randy Holmes-Farley, “Reef Aquarium Salinity: Measurement and Control” (Reefkeeping Magazine); Tropic Marin, “Salinity and Conductivity in Seawater” technical notes; Neptune Systems, conductivity probe calibration guidance.
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