It fluctuates probably due to the fact of evapouration.
From what I learn in school, pH is power of hydrogen. Which means how many free floating hydrogen atoms are in the system.
Acidic pH is lower, because of the fact in all acids you need that hydrogen atom. Eg. HCl hydrochloric acid.
Basic pH is higher, because of the fact you need a hydroxyl group rather than hydrogen. Eg. NaOH sodium hydroxide (which is not a pure hydrogen atom attached)
So when the water evapourates you'll have more of a concentration of hydrogen atoms which will make the water more acidic. When you top off it dilutes everything which will raise the pH.
Thats why I think the pH might fluctuate during the day.
But if the water source your tank waters comming from is to acidic it wont matter what you do, unless you add some hydroxide molecules to the water lol to make it more basic.