Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Vinegar is easier to find, and not a problem to wash off the rock if you decide to use it.
Another way to test is to put the rock in a bucket of water and test GH, KH, pH and TDS every few days for a week. If you will be altering your tap water to be more acidic, then do that to this test so the rock is being tested under conditions as much like the tank as possible.
Highly reacting rocks would show changes in these tests within a few days.
Slow reacting rocks might show a very small change over a week, but that might be OK, if you will be doing weekly water changes.
Non reacting rock would not show any changes if you tested it much longer than a week.
I have heard that lace rock is one of the limestone types that will add minerals to the water and raise the pH. Some of these rocks are much more reactive than others, though.
Perhaps the person who had it before used it in a hard water tank. Any of the limestone types of materials will not affect that sort of tank, or at least very little, and are actually good. If the water in a hard water tank starts getting less alkaline then the rock will react, releasing some minerals into the water and bringing the water chemistry back to what those sorts of fish like.