I bought a small bag of DE for the sandbox I'm building from a local feed store and then some of the threads I'm seeing on here talks about 'food grade' DE... what is the difference and does it matter that much when it is for a sandbox and not around their food? I'm sure they will eat some of the bigger grains/small rocks in the sand for grit, is that why 'food grade' is important? Is regular DE okay? Also, how much would you use in a sandbox, would you use it at the bottom of the sand, the top of the sand, mix it in with the whole depth of the sand? Thanks!!