If it has built up over time, and caused your tank to have high nutrients then that is possible, but my point was that 7.8 is not a problem.
As easy test is to take a good sized air pump with a ceramic or glass diffuser (not a cheapie stone), and let it run for 24 hours, then re-check your pH (at the same time of day you made the first test). If your pH has risen then the cause is excess CO2 in the tank. If the pH doesn't rise, then it is something else.