I checked my water temp next to my Apex Temp Probe with a NIST Certified +/- 0.4 Degree Fahrenheit thermometer as well as two other thermometers and came out with an average reading 1.5 degrees lower than the Apex was reporting. I trust the NIST Thermometer to be closer than the Apex out of the box due to the NIST Certification that my thermometer carries [and is still valid]. This means effectively that when I had set my aquarium to run at 78 degrees it was really running 76.5 which is a bit on the low side. Granted I'm likely still not exactly 100% accurate - but I'm going to be much closer than I was. I turned down my temperatures a bit and will up the temperature from 76.5 to 78 over the next 12 hours or so as not to have a sudden change.