Here's a handy use for a pair of neodymium magnets and a little CNC-magic bracket.  Combine them and you have a simple, secure way to keep probes in place and at the correct water depth.  The magnets will hold through 3/4" glass, accommodating nearly every sump out there.
 
The bracket slots can hold our ATO sensor tube, and the magnet mount comes with custom rubber bushings that accommodate either 1/2" probes, 1/4" poly tubing (commonly used with dosing pumps) or any combination of the two. The flexibility of the design allows for all probe styles including the Neptune APEX temperature probes.

4-Slot Dimensions: 1-3/4" front to back, 4" wide.
External magnet half is 1/2" thick.