This probe will measure salinity in the tank and is compatible with all Neptune Apex models. They can act as replacements or can be attached to PM2 among other monitors. This probe will provide alerts to your computer or phone when programmed. 

The Conductivity Probe is a product in the Neptune Apex AquaController System line and is compatible with all APEX systems (Apex Jr, Apex Classic, Apex Lite, Apex). The Neptune Apex System is probably the most technologically progressive comprehensive aquarium system on the market. Featuring full wireless compatibility, customization, and control, the Neptune Apex System is unrivaled by its competitors. This system is the future of the home aquarium. You can set up the system to send you email alerts on any number of factors, alarms, or even phone notifications to monitor tank conditions. The system is fully customizable and will receive regular software and firmware updates to ensure continued efficiency and to keep the system as up-to-date on current trends as possible. All products are interchangeable and customizable and will fit into other products seamlessly with no programming required.