Most all rock will be pH neutral. Some rock like Limestone or some volcanic rocks may have issues. Also some soft stones like types of sandstone, or stone with heavy minerals like salt or iron, may also have issues. But Don't think you have anything to worry about with river rock. Especially the stuff used for aquariums because aquariums need to be kept in the right pH range also.
But it should be easy to test. Simply get a small bag of it, wash it off to get rid of all of the dust. Dump the washed rock in a bucket, pour in some water that you tested the pH of first. Then check it every couple of days for a week or two, and see if the pH changes.
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