What kind of gravel do you have in your driveway?

I know around here, gravel driveways tend to be crushed granite and limestone. Either of these I would expect to steadily raise your PH.

I dont know of any type of stone that will push PH down, off the top of my head, but it might be something to look at.
Sorry if I am way off base here, but when I saw "gravel from my driveway" it just stood out.

Also, did you make sure to clean that gravel fully? Stones in a driveway tend to become contaminated with all kinds of things that are bad for both plants and fish.

-Larry