This is how it was explained to me. You paid for the tank. Think of it this way, if you add the rock first, then the hard, often times sharp rock is in direct contact with the tank, possibly putting stress on the glass or acrylic. Try to add some sand to for a shallow bed, then put your rock in on top of it, then add whatever additional sand you want. This will help to cushion the bottom surface of the glass as well as giving you support for your rock. But of course, when it all boils down, your tank, your choice, just be willing to stick to your guns. LOL.