I used expanding polyurethane foam over egg crate to create the structure. Cork bark was embedded in the foam (technically, the foam was added around the cork bark). The foam was then covered with Hygrolon.
The filter mats are there as a drainage layer, and to maintain aeration beneath the substrate. The bottom of the tank was also drilled to allow water to drain out freely.
This tank rocks
Super cool work. Can you list what materials you used to create the hardscape and describe the filter section please