shrimp, crabs and snails are all inverts. They can be sensitive to the spike of ammonia and especially nitrites. I've used the so called bagged live sand before and had excellent results with it but it's not really live sand as we would think. Instead of being full of pods and worms etc it has bacteria that aids in the cycling process. To get nice live sand beg a cup or two of live sand out of an established aquarium. Get some from the LFS, get some from a neighbour the more places you can get a cup or two the more diversity of life you will have in your sand.
Doug
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