Eheim - I don't get it. Right out of the box I put heater in tank let sit for about 30 min. Set to 78 degrees. Temp is always about 80 - 81 degrees seems to fluctuate. Wanting this to be accurate I tried the recalibration instructions. Now water temp is 77 and I have dial set at 79 and light won't come on to heat water. Anybody have any luck with these things I have been messing with this thing for about a week and can't get it right.