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Coral and shells will raise the pH of your tank. Depending on your current pH, how big the tank is, and how much crushed coral and shells you put in, it might be ok, and it might not.
Depends on the shells, depends on the buffering capacity of the water, but shells and coral skeleton contain calcium carbonate which as it dissolves increases pH. Every tank is different but it is something to be wary of before adding them to the tank.