One of three rocks holding down the huge piece of wood in my 125G tank. The rocks are also an integral part of the aquascaping.

The rocks used in my 125G Tropheus tank are identical to the ones used for the real rock background in my 240G tank. They are Ohio top rock - a type of sandstone that is very inexpensive in my area, has a nice natural look, and can easily be drilled using a standard masonry bit. This allows the rock to be attached to wood using drywall screws that can go through the rock into the wood, or through the wood into a plastic anchor (dowel) that sits in the rock.

Masonry drill bit, plastic anchors, and drywall screws.