Unless you are raising certain patterns of Koi, general hardness (GH) is pretty meaningless. My water comes from a well and the hardness is literally off the charts.
What is important is KH- Carbonate Hardness. KH is what buffers the water and prevents a pH crash. Limestone can be used, as well as crushed Oyster shells. Or Baking Soda. The myth that using certain kinds of stone in the pond comes from the fact that the pH may be naturally raised in that situation... but the trade off is better buffering. pH crashes kill fish, not consistently high pH. It's more important that the pH is stable, and buffering the water accomplishes that.