It really depends on how heavy all your rock is. From everything I've read it's really personal preference. I've heard of people putting 150lbs + in their tanks with nothing and never had any problems. However, if you want to put something in, you can use eggcrate. It's also known as a light diffuser, you can find it at the hardware store in the lighting section. Its a plastic grid, usually in 1 x 1 squares, that you can cut to size.