Guys I got so lucky. Didn't know that Big Al's usually doesn't take equipment back that is already used. But the owner had mercy with me and exchanged the fluval for a marineland submersible 400 watt heater.
Turns out that the 300 watt really wasn't enough for me. The new heater works perfectly fine and holds the temp consistently at 80F for the last 24 hours.
I got an old fashion floating thermometer as well. So no more confusing temp readings.
Bottom line: the fluval heater is a good one but it could be that the watt is not enough for your tank. It really depends on your room temperature. Learned again something new. unfortunately they don't have 400 watt heaters.
I am very happy with my new marineland one. Fish is happy and warm