A long, long long time ago I had some bright neon pink and green gravel for my first fish tank. I'd forgotten about the atrocity only until yesterday when I was looking at the garden and lo and behold there was a single green booger rock sitting in the middle of the roses.
Personally I don't use gravel, just because a bare bottom is easier to clean. Decorative, no, more practical, yes. But if I did get some kind of substrate it would either be sand and river rocks or black gravel and river rocks since I'm liking natural looks more and more. I think I might do an overhaul of the large tank though ... the big pink plastic log thing is totally bugging me. And though I have no green thumb whatsoever, I might try for some live plants. Money though .... I have none of it .... *sigh*
I would do a zen theme - a nice background with Asian lettering on parchment paper, sand substrate with a few larger, smooth rocks, a small rake (going for a zen rock garden), a buddha, and some bamboo, java fern, or water wisteria.