Depending on what is in your tank, PH can bounce around .5 in a 24 hour period.
As Oxygen is generated during the lighting period, PH typically goes up. At night when the lights go out, PH starts to drop as the oxygen level drops.
A closed house during the winter can contribute to the ph issue but the 7.8 you are talking about is a pretty typical number for an early morning reading.
Keep a record of the PH (with the time) Note the normal swings for a couple days and then air out the room more than normal and see if it changes.
A PH meter is a lot easier if you can afford one. I would get something like a Pinpoint that stays in the water all the time. The pocket style testers are a pain in the tail but again are only half the price
How good is your surface agitation? if it is too calm, gas exchange may not be very good. Try redirecting a powerhead to put a good ripple on the surface.
I maintain a PH of 8.2+ with a kalkwasser doser that runs at night when the lights are out.