You do need to be concerned with what is in or on the substance(s) you use for mulching. Stuff gets into the soil, for one thing. For another, I heard of an instance where a guy put a lot of bark in his flower beds and some strange mushrooms grew; his dog ate one and then totally tripped out; it ran around crazily for a half an hour, and then dropped dead. Rocks are not likely to have that effect. However, if you put strange rocks into your fish tank, you may get all sorts of oddities. Especially if you get the rocks from under water somewhere.
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