What time of day did you test it? 7.8 could be normal for the morning, it may go up to 8.1-8.2 durin the day. Test it just before lights on, in the middle of the day, and just after lights off, this gives you a more accurate picture of you pH levels. Simply a water change is the safest way to raise pH, if that doesn't help then some of the buffers are good, I prefer any buffer be slowly dripped in the tank, preferably in the sump.