I've been known to be less cautious than some, but here's my opinion anyway:
Unless it's been in a copper basin, catching gutter water from copper gutters (if it is rip those off and take them to a salvage yard for me pls) then you should be ok. I agree with Mario that I would re-cure them, because likely they have had a lot of die off from being pulled out of the tank and flooded with fresh water. Keep in mind that you are essentially picking up base rock, and the price should reflect that.
If you are worried about it, set up a nano with one of the rocks in it before you use it. Test it out.