I hope nobody minds me bumping an old thread, but I just wanted to say that I saw this back when I was lurking, and I love the Metroid tank.
I'd love something like that and I think it was the first tank that showed me that having no gravel could look good, something I'd worried about for some time.
I'm afraid I'm nowhere near as artistic and the best I've come up with is a couple of soil-pipe cutoffs, but the Axys seem to like them. They used to have loads of weird pipe fittings/joints from when I was a labourer that I'd swap out every now and again to make interesting tunnels, but my mum gave them to the builders that did out the bathroom, even though all our work was above ground and the bits ended up in a skip.
Anyway, congratulations on having such a great tank.
I see it mentioned elsewhere in the search results and I'm interested how it worked out, so when I've got a minute I'll browse some more, but I'll try not to bump too many old threads, I promise.