pH fluctuates through the day. The net consumption of carbon dioxide during the daylight hours, leads to some aquariums to become deficient in CO2 during the day, and the pH rises.If you are having dramatic fluctuations you can aerate the aquarium water to degass excess carbon dioxide.
To make a small correction you can use a buffer like TM Triple Buffer or use a good salt with a naturally high buffer capacity like Tropic Marin or similar.