I disagree. Beach sand, if properly washed (to remove salt and debris) and sanitized (bake it) can be fine. The crushed shells will raise the pH slightly, but will also make the water harder (which isn't necessarily a bad thing). Most new tanks have a problem with too acidic water, and calcium carbonate is one way to counteract that. Anaerobic pockets will happen with any substrate if it's a thick enough layer. I suggest only using 1/2 inch of sand anyway. Any more than that is pretty unnecessary.