So my Eheim Jager heater 75w for my 20 gallon tank has been maintaining a pretty stable temp for me around 76-78 degrees. My light heats my tank during the day at 78 degrees, then when the lights go out my heater will maintain it around 76-78. Last night I shut my lights off at around 9pm and the tempurature was normal. Then I had a late night and got back a little after 1am and looked at my digital thermometer for my tank. It was at 82 degrees and the heater was still running!!!! This scared me as I thought the heater had stuck and the tempurature would keep rising. I quickly unplugged the heater, then went to bed. 30 Minutes had past and I somehow remembered that I had a cheap back up thermometer in the back of the tank, thinking that maybe my digital heater wasn't working, so I went to my tank and looked at the back up and it read that the temp was 77?!?!?! Then I looked at the digital and it was going from 77 to 76 to 78 in the matter of minutes... I gave it a go and plugged the heater in, then fell asleep. I woke up with the temp at 77. Now I am not running my lights just to see if everything ok whether it is my heater or my thermometer. What do you guys think happend? Do you have any advice on what I should do? I am very concerned because of my live critters in the tank.