Hi all,
new here, but saw this and thought i'd give you some insight.
you're probably experiencing carbon dioxide dissolving in the water. yup it does. this also alters the pH of the water very slightly too.
carbon dioxide and pH are closely linked; and works similarly to your blood. it's a 'buffer' system, and as pH changes, so does the amount of dissolved carbon dioxide, etc. it's in a balance. in your blood, actually, the change clicks your heart/lungs to race faster when there's higher c02 dissoloved.
essentially if you want to knwo more, look into the buffer equation for carbonate buffer systems. something like:
H2C03 <--> HC03(-) + H(+) <--> C03(2-) + 2H(+)
notice the left side has NO H+, the right side has 2 of them (the more "H+" you have, the higher the pH
there's your science.
-m