Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
That seems unusual - have you checked the temps both day & night several times ?
If your tank is uncovered, and the tank room temp is kept quite low during the night, I can imagine a slight drop overnite, but that seems to be quite a large drop.
Perhaps your heater is a problem - I run a 300 watt calibrated Eheim Jager on my 75 gal discus tank, set @ 84, and it doesn't move more than about 1/2 degree at any time, and it's an uncovered tank.