I just redid the rocks in my tank last night. My goal was to make a lot more caves.
I have two different types of rocks in my tank, flag stone, and purple/green slate.
Both of these rocks break easily with a hammer so I was able to get the exact sizes I wanted for all of the peaces. I have white silica sand along the bottom of my tank, so I was sure to place the rocks directly onto the glass rather then on the sand. Anyhow, what I did, was broke all the flagstone into peaces about the size of my fist. I then placed them over the bottom of the entire tank, leaving a couple inches betwen each peace. I then go large peaces of slate and coverd this. SO my bottom layer of rocks is very solid and stable, but also has created dozens and dozens of caves. From there I want and continued to stack the slate untill it came just past the middle point all the way across.