pH changes are due to the starting water conditions in the tank. Specifically the carbonate level (pH buffering component).
The white streaks on the stone is calcite. A hard form of calcium carbonate.
In acidic conditions with low carbonate levels the calcite will slowly dissolve. This will increase the pH higher. This can kill delicate species that need acidic conditions.
If the water has a neutral or basic pH with higher carbonate levels to start with, none or limited effect will be noticed.
