Understanding reef tank salinity and taking accurate readings

Salinity describes how salty the water is. Keeping it appropriate and stable is a basic part of marine aquarium care, but readings are only useful when you understand the instrument and the units it displays. A careful measurement routine is more valuable than repeatedly adjusting the tank to chase a number.

Know which scale you are reading

Marine instruments may report salinity in parts per thousand, practical salinity or specific gravity. Specific gravity compares the density of a sample with a reference and depends on temperature and the instrument's calibration. Do not treat a specific-gravity number and a salinity number as interchangeable, or assume every refractometer uses a seawater scale.

Choose a target suitable for your animals and husbandry plan. Many reef aquariums are maintained near natural seawater salinity, around 35 parts per thousand, but this is not a reason to make an abrupt change to a tank with a different reading.

Follow the instrument's calibration instructions

A refractometer, conductivity meter and hydrometer have different checking procedures. Use the calibration material and method specified for your model. A standard suitable for one instrument may be inappropriate for another. Keep standards clean and within their stated usable life, and never pour used solution back into its bottle.

Clean the sample surface or probe as instructed, allow the required temperature equilibration and avoid bubbles where they affect the reading. Write down the instrument, units and result so future comparisons make sense.

Use the same sampling routine

1. Check that the aquarium is circulating normally and any top-up has mixed through.
2. Take a clean sample away from a recent freshwater addition.
3. Read the instrument using its specified procedure.
4. Repeat an unexpected reading before making a correction.
5. Check against a suitable standard or a second reliable instrument if the result still looks wrong.

Find the cause of a change

Evaporation, saltwater removal, salt creep and top-up faults can all affect the balance. Replacing evaporated water normally calls for fresh purified water; replacing removed aquarium water normally calls for appropriately mixed saltwater. Keep those jobs separate.

If salinity has moved significantly, confirm the reading and plan a controlled correction suited to the livestock. Our evaporation and top-up guide explains the everyday distinction, and a maintenance log helps reveal trends.