Just a whole load of wood n rocks I reckon! You would need so many plants. I still want it though think how many khuli loaches and neons I could fit in there!! x
And, of course, your floor would have to be absolutely perfectly minutely down-to-a-gazillionth-of-an- inch level, with no deviations whatsoever. Mind you, if you can afford a tank like that, you'd probably rip up your floor and start again.