Plug this Neptune Systems Leak Detection Probe into your Neptune Systems FMM and then place it on any surface where you want to detect stray water. Upon contact with water this probe will communicate to your Apex and alerts or other actions can then take place. Time is so critical when it comes to water on the floor so it is very common to place two of these inside the stand and two outside the stand to have the ultimate protection against a leaky aquarium. Note: The LD-2 will only work with the FMM and cannot be used with the ALD module.