I might be over thinking but I got my back up heater in and it is an Eheim Jager 300W. I need to calibrate because I only want it to come on if the temp goes below 77F. I am unsure about what thermometer I should trust. I have a Coralife digital. I tried calibrating it using the ice water method. I filled a large container with crushed ice and tap water, stirred it and put the probe in it. The thermometer read 30.5F so that should mean that it is off by 1.5F, Correct? My Apex says the temp in my tank is 78F and the digital says 76.8 so that is about right, correct? How much should I trust the Apex probe?