The Neptune Systems Apex Advanced Leak Detection Solid-Surface Probe (ALD-P2) is designed for use with the Neptune Systems Apex Advanced Leak Detection Module (ALD). This Solid-Surface Probe (ALD-P2) is designed to be placed on solid surfaces. Alternatively, you could use the Low-Profile Probe, which is a tad smaller and slides easily under a carpet that is behind or next to your aquarium.