Medium and shelves to hold planters

[Shanna Tolley]

Having a hard time finding clay medium locally. Don't want to pay the extra fee for shipping. Would small gravel from like a Lowe's be ok? And how would I know if it doesn't have marble or anything else in it that would be bad for the fish?
Also, I know this will be heavier than clay. As long as I have chains on the front side of my shelves above my 55 gal tank and a lip on the wall for more support on the backside the shelves should hold two rectangle planters right? Not much of a carpenter haha. Here's a photo.  fishtank%203.jpg Thanks