Welcome to AA!
Open up some windows in your house....but looking at your location that probably isn't an option this time of year. Your pH at 7.8 is OK. It's normal for the pH to drop during the winter in colder climate areas where your house gets closed up and the air in the house has a higher co2
level than during the sunmmer when the windows are opened.
I would suggest not adding buffer to try and get the pH up. It will mess with your alk
levels. If you really want to try to raise the pH, you need to get fresh air into the system and that can be done if you have a skimmer. You can run an airline to the outside and let the skimmer pull in fresh air.