You can purchase temperature detectors for use with the unit. They are not water proof. I took a 2 ft piece of 2" PVC tubing and glued a cap onto one end. I put the temperature sensor into the bottom of the tube, then put some aquarium gravel over the sensor (you could put it in a ziploc bag as well I suppose before putting into the PVC). The gravel weighs down the tube, and creates a volume of material that will be slow to change temperature. I then put the PVC tube into the sump of my tank. After that I set the temperature limits on the Sensafone alarm to go off at 73 F, and 85F. Too hot, or too cold Sensafone gives me a call.