Salinity is the measure of dissolved salt content in water. The concentration of salt in an aquarium is typically expressed in parts per thousand (ppt) or specific gravity (sg).
It is extremely important to keep the salinity level in a marine aquarium stable. That is why the aquarium refractometer is a tool found in every serious saltwater aquarium hobbyist’s toolbox.
The specific gravity in our oceans vary, although higher salt concentrations are often found around coral reefs. For a fish-only (FO) or fish-only with live rock (FOWLR) aquarium, the specific gravity should be in the 1.020 - 1.025 range. For a reef aquarium, maintaining a specific gravity in the 1.023 - 1.025 range is perfect.
A standard refractometer resembles a small spyglass. To use a salinity refractometer, a few drops of water are placed on a glass pane and then covered. Look through the other side toward a bright light source (like your aquarium) to view the water sample. The salinity reading is where the colored half of the viewfinder stops. New seawater refractometers should be calibrated before use and occasionally thereafter to maintain accuracy.
Milwaukee and Hanna Instruments digital refractometers are another option. They are twice the cost of a normal seawater refractometer but they eliminate all the guesswork. Rather than relying on your own ability to decipher the salinity measurement, a digital refractometer will provide you with a simple digital reading in ppt, sg or the newest scale standard, PSU (for practical salinity units).
Although simple (and inexpensive) hydrometers can measure salt content, we always recommend using refractometers since they are more accurate and easy to use. If you questions about refractometers, please contact our aquarium experts
for free and friendly advice. You may also want to bookmark our Reef Tank Parameters Chart
and read How to Measure Salinity in a Saltwater Aquarium
to further your knowledge.