Good advice TR. You certainly wouldn't want to pollute your substrate before you even set up the tank. Besides that, if your rock was really nasty and sheds a lot of gunk, you'd still have the option to drain the tank and start over. It'd be a lot simpler to vacum out the sludge in a bare bottom. Then when the rock cures you can simply lay it in a tub while you set up the substrate and arrange the rock work. Get a large rubbermaid type tub and fill it with about 10g of RO/DI water. When your rock arrives scrub it down to knock off the large chunks of waste before adding it to the tank. This will help prevent fouling the tank water to quickly.