What will the effect of limestone be in already hard water?

Take some vinager to the place if the rocks your looking at bubble don't buy them if they don't you are safe to use them.

If your water is already that hard calcium carbonate rocks won't make the water harder at all, Calcium carbonate only acts as a buffer when the hardness and ph of the water drops real low, That's why us reef guys don't have to use sand in our saltwater tanks because it really doesn't do anything for buffering unless the ph drops to low(which should never happen), Now if your after real soft water then I would stay away from any of those rocks but they won't affect anything if your water is already hard.

Not a problem. As noted by DSP the calcium carbonate types of rock will only dissolve in water that is acidic to neutral. It does not dissolve very fast in harder water. Just enough to keep the parameters stable.

This is why it is well thought of for marine tanks and hard water tanks such as for Rift Lake Cichlids and certain other hard water fish.
If the water ever does change, the rocks change it right back to where it should be for these types of tanks.

yep, my water has a ph of ~8 and very hard so even with rocks that are said to raise ph/water hardness I don't ever see a difference

Whoop Whoop!!!!! Thanks!

I've never trusted the vinegar test, not strong enough.