This is what I use has a wireless Indoor/Outdoor Thermo-Hygrometer sensor. Doesn't need a probe just place the sensor in the tank and you get very accurate hygo/temp reads. Way more accurate than any temp gun for racks as well since you lose a few degrees once you open a tub, aim at just the right distance and wait for the temp gun to read the hot spot.

Link:
http://www.partshelf.com/acu-rite-00611.html

This is also sold at Lowes if you have a local store.