You can test rocks by pouring vinegar on them (acid) if it bubles or reacts this is not a rock you want in the pond as it will increase your hardness and ph.
PH in a pond fluctuates by time of day with morning being the lowest and night being the highest, due to plants giving consuming O2 at night and using up CO2 during the day.
It's normal for a pond to swing the PH. Mine has gone from 8.0 at night to 7.2 in the morning.
I wouldn't add any chemicals to a pond to lower PH (lowering is the hardest, raising ph is easy). You will end up having a ph crash or bouncing PH. It takes a lot to buffer water to a stable PH with any buffer there at all you won't be able to lower it and will spend $$$ adding chemicals to lower it. It's not worth it.
The best water for fish is stable water even if the ph is higher than you would like, the fish prefer water chemistry that doesn't move alot.