As Rich said...you are getting ballpark pH. You need a high range pH test.
Your pond is not cycled. It likely will not be for another month.
You can either get Prime or Safe to bind ammonia and protect from nitrites (dosed for the full pond volume every 48 hours) or you can use another ammonia binder (amquel, etc) and use the salt to bind the nitrites.
Your issue is not the rocks...it's that your pond is not cycled and you have ammonia, incoming nitrites and likely issues w/ swinging pH.
You need a High Range pH test, a KH test kit, Prime/Safe or another binder + salt.
Andrea
Koi Health Care Committee Member