1) I would get it analyzed for elements that might be toxic in tank such as Cu(there's a water kit), it's just good practice depending on what you'll be keeping in tank. Just experiment a bit, if your shrimp and snails start dying, it could be the cause (I've actually had this happen to me but I suspect it was from using too many root tabs)
2) How big is your tank? I would use thin, say 1/8", slabs or pellets of it as your "bottom" substrate followed some finer grain, and finally a coarser river gravel or sand, 1:1:2 ratio of clay, fine sand, and coarse river gravel or sand.
3) The clay is also good for nitrogen reducing/fixing bacteria.