The FMM (Fluid Monitoring Module) is a multi-purpose module that, with the help of various accessories, enables your Apex to do:

Kit Includes

FMM (Fluid Monitoring Module)
The FMM (Fluid Monitoring Module) is a module that connects to any Apex (Apex(2016), Apex Classic, Apex Lite, Apex Jr.) via it’s AquaBus port providing four multi-function monitoring ports that can be used for flow sensors, water-level sensors, leak detections sensors and possibly future products as well.
1" Flow Sensor (FS-100)
This flow sensor connects to the FMM, monitors the flow of water through the device, and then reports it back to your Apex via the FMM. This sensor is most often used to measure the flow in the main return line of a standard-sized aquarium. The FS-100 will work with flow rates from 150-1500 gph.
Two 1/2" Flow Sensors (FS-50)
This flow sensor connects to the FMM, monitors the flow of water through the device, and then reports it back to your Apex via the FMM. Two of these are included in the FMK. These sensors are perfect for attaching to devices such as media reactors, biopellet reactors, etc. The FS-50 will work with flow rates from 30-250 gph.
Two 1" Unions
To connect your FS-100 Flow sensor, Neptune Systems has included in the FMK, two 1" BSPT -> 1" Slip PVC Unions.
Two 1/2" Adapters
To connect your two FS-50 Flow sensors, Neptune Systems has included in the FMK, two 1/2" BSPT -> 1/2" Slip PVC Adapters.

Flow Monitoring Kit - FMK-I - Neptune Systems