salinity in plain terms is the concentration of salt in aquarium water more specifically salinity is a measure of the dissolved sodium ions in the water and it can be measured in parts per thousand pptmost saltwater aquarists measure the salinity in their aquarium using a hydrometer which measures the specific gravity in the tankith the proper equipment and regular water tests it should be easy to maintain a constant level of salinity in your saltwater aquarium if you take the proper precautions you can make sure that the environment in your tank is as close to the natural environment of the ocean as possible