What's your alkalinity? Also, have you tried opening a window for a while and re-checking ph? If your co2 levels are high then your ph is going to be low... Opening a window can help with this... Although having a window open all the time isn't practical... Some people have had success running an air tube from outside to their skimmer.
7.8 is low, but it's not terrible. Are you seeing indication of stress from anything? My ph tested 7.8 for over a year, but I had good growth so I gave up on trying to 'fix' it...