The AquaMaxx Probe Holder is constructed of high quality cell-cast acrylic and utilizes powerful rare-earth magnets that work on aquariums as thick as 1/2". Features two smaller holes for pH/ORP probes and two larger holes for larger-diameter probes; such as the Neptune Apex Temperature Probe. The unique box design with 3 contact points also ensure that the probes are ultra secure and upright when installed.