Low Ph and an airstone.....

I everyone, I'm new here and I do plan on adding all my tank info in
the next topic but for now I just want to know why an airstone is
recommended in a reef tank to raise the PH but I've so far never seen a
pic of a tank with bubbles in the background. I have problems with my
PH and have read several articles in this group regarding this problem
and lots of answers revolve around an airstone, does it go in the main
tank or in the sump?
Louise