Neptune Systems PM2 Expansion BoxNeptune Systems PM2 Expansion Box Real time Monitoring of Salinity in Your Aquarium Maintaining consistent salinity in your aquarium is key to the well being of every organism inside it. And, as busy as we sometimes are, we dont even realize our parameters are out of whack until its too late. With this module, your Apex will be constantly monitoring the Salinity in your tank and will let you know the minute it is trending outside of the safety zone.