05-08-2013, 11:44 AM
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Mitch is referring to calcareous rock, like limestone, dolomite, marble, lava, etc, that contains calcium/magnesium and will react to an acid by fizzing. Such rocks release calcareous mineral into the water, which is not going to hurt hard water fish or shrimp, but if you have soft water fish this could be a real problem.
As for other substances, you can't tell. I use "river rock" in a couple of my tanks. This is rounded bits of rock (smooth) that you can get in all sizes from gravel up to boulders. It is rounded because it has been tumbled around by a river and worn smooth over time. This is generally safe. Some people collect this from a river or stream, but I prefer buying it from a landscape supply place as it is very inexpensive and I can select the sizes and uniform colours.
Porous or soft rock like sandstone is not a good idea.