Partial water change is your quick fix.
What size tank? How much live rock? How much live sand? Do you have any macro algae? How old is the tank? Do you have an alk
/carbonate hardness test kit? Check your alk
, if your levels are too low, it will drag your ph down. Really, 7.8 isn't drastically low, it's still within the acceptable parameters, although any lower than that and your inverts will suffer. Ph swings are more worrisome than maintaing a perfect 8.2. Do you have a refugium? Is it lighted? Does your aquarium have a glass cover over it, or is it open to the air?