Using a Refractometer Correctly

This guide covers aquarium refractometer salinity with practical steps for home aquariums. Refractometers beat floating hydrometers for precision — but only when calibrated before every test session.

For related reading, see Hard Water Fish and Rift Lake Chemistry, mastering aquarium pH and Controlling Nitrates Long Term.

Why Refractometers Win

Optical refractometers measure how light bends through a water sample — more accurate than swing-arm hydrometers affected by bubbles and magnetite dust.

A quality marine refractometer with ATC (automatic temperature compensation) is standard reef gear.

Calibration Steps

Place a few drops of RO/DI or calibration fluid on the prism, close the cover, and adjust the blue line to zero before testing salt water.

Skipping calibration causes chronic salinity errors — fish adapt until a water change shocks them with wrong salinity.

Taking a Reading

Use tank water away from fresh top-off layers — mix sample if needed. Spread thin on prism, read where blue meets white at line crossing.

Record specific gravity (1.025–1.026 common) or ppt (32–35) consistently — do not mix units log to log without conversion.

Common Mistakes

Wet fingers contaminating prism, testing cold sample on non-ATC units, and dried salt crust on cover plate all skew results.

Wipe prism with RO water and soft cloth after each session — scratches ruin accuracy.

Salinity Targets

Most reef keepers mix new water to 1.026 for display stability; fish-only can run slightly lower. Match change water to tank reading within a point.

Evaporation raises salinity — top-off freshwater only, never salt mix, to compensate daily loss.