Oh, and the cheapest place to pick up slate is your local landscape supply company. PRetty much all the rockwork in the above tank was just over 100 pounds. Cost me about $30. There is a piece of slate I had already on the left, the piece of granite and the piece of petrified wood in the bottom middle. The rest I bought at a landscape supply company.
When I told the guy what I was looking for he helped me find ones with cool color patterns. All the blocky gray stuff in the tank has dark wavy striations in it that doesn't show up well from a distance. Look at my avatar. You can see the striations behind my OB....