I heard krylon(for outdoor furniture) works well and comes in multiple colors. You can buy huge sections(4'x4') of 1,2,3, or 4" thickness foam to carve out and create texture.
Another awesome method I have seen is the PVC do: PVC is shaped with elbows, T's, and angled connectors, then covered with the pond foam, followed by pool coat, followed by krylon... the pool coating comes in different colors, so you might be able to skip the krylon if you're ok with a solid color.
BTW: the PVC can also be used in the plumming of the tank... multitasking decor is so cool... a tree root system that filters the tank? almost like nature!