Can't trust only one Thermometer

[Frogger]

Originally Posted by gregzz4

I have 5 of the Hagen floating glass therms which I use to check things. 2 read 1 degree higher than the other 3 so just split the difference. I also have a couple Big Alert Digital units and all match each time I check. I use them to calibrate my controller probes.

On a side note, I run my tank @ 78.5ish. I'd be hesitant to run a controller at your 84F. Hard on some fish and many corals. Plus depleted O2.
This will also give you more algae and cyano issues.

I originally set the finnex controller at 78 degrees but my water temperature was less than 74 degrees and the heater wasn't on.. I raised it to 80 and the temperature came up to about 75 to 76 before the heater turned off. I had to raise the temperature on the finnex controller so that it reads 84 before it would shut off at 78 degrees. I have been running it at this setting for the past couple of weeks and I check the temperature 2 to 3 times a day with the Vee Gee digital thermometer. It only fluctuates less than half a degree.

This means the controller is calibrated way off. So don't trust the reading on controllers.