Open your windows, oxygen will increase ph if there is excess buildup in the house. I would also be testing your alk
. Adding buffer will raise your alk
fast if your not careful.
7.8 is ok and will not harm anything however long term it can become a issue. Alk
and ph go hand and hand. Excess co2
is also the main cause of depressed ph.
Also when you do a waterchange and the ph is 7.8 in the tank if your doing a 20% water change with 8.2 water then the tank water after the change is not even going to be 7.9 that is why you noticed no change. If your alk
is low then it will take a few water changes to get it buffered where it belonga for your ph to become 8.2 in the tank.