Hopefully last PH Question..

First I would like to say THANK YOU to Stinky. You gave me very sound advice Friday which I followed & I am happy to report you were right (as if there is ever a doubt!). She looks much better today. And of course 1 more ph question. lol... So I was using the ph down cause of the tap water, but for the clone I decided to buy a jug of water and not chance any young growth problems. It phed at 7..Yes this would be a breeze. The gallon is about 1/2 gone and tonight I remember reading about some folks having problems with their ph when the water sat out so I decided to ph my jug again and holy guacamole 8.5 or higher..blue/green on the liquid tester. Now it has been open about a week, just sitting on the counter with the lid on it. So why would the ph go up so much?? Any thoughts on this please feel free to share. Oh yea & I am going back to tap. It is high, but not as high as jug sitout. And of course tap has all the other good stuff in it. Lesser of 2 evils as they say.