Why and How does pH level change?

Over the past weeks I've been doing an Ecobottle project in my grade 10 science class . We've been observing the pH levels in our tanks which are now sealed off to observe if our guppy , snail and aquatic plant can support their own ecosystem.



As i observed the water's pH level , ive noticed its been around 6 - 7 pH , which is normal from what i've been told .



Oh, and the water was just tap water.



So , now i just need an explaination on why pH levels change, and how they do . thank you ! :DWhy and How does pH level change?
Carbon dioxide lowers the pH. I would say pH is low at night and higher in the day time, because in the day time plants remove CO2 from the water. Why and How does pH level change?
pH is how acidic / alkaline something is. 1 is the most acidic, 7 is neutral and 14 is the most alkaline. tap water is neutral (7)