Two different ALD probe designs are available from Neptune. One ALD probe is low-profile and slides easily under the carpet behind or next to your aquarium (pictured). The other probe style is a tad larger and is designed to be placed on solid surfaces. This probe is perfect for inside the bottom of your aquarium cabinet. And, we add extra weight to this probe so that it stays firmly against any surface you set it on. The ALD and the individual leak detection probes are sold separately so that you can mix and match the probes you want to use (up to four per ALD).