Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Put one of the rocks in a bucket of tank water, or RO.
Monitor the GH, KH, pH and TDS for at least a week.
If there are any changes, then continue testing for longer. If there are no changes in a week, then the rocks are probably fine, though running the test out for a couple of weeks would be better.
If there are changes, but very small ones, then you will be resetting the parameters every week (with weekly water changes) and the rocks are likely to still be OK, but maybe not for something as delicate as wild caught soft water fish.