Here's something I've been working on for quite a few months! Last year I started this terrarium project around April, with a handful of cuttings from my various house plants (pothos, philodendron, etc.) in jars of water. I knew from the beginning I wanted to put some sort of reptile/amphibian in it, and was originally thinking frogs. But, it just so happened in May that :epicwang:'s workplace had a bunch of geckos that needed homes, so I added a gold dust day gecko into the mix. :3 If you look closely, he is in the picture!

All of the plants (except for one) are rooted directly into a layer of unglazed, washed landscaping gravel; I specifically chose plants I knew would grow well in just water, with the exception of the red-leaf philodendron (the red vine at right), which is rooted into a small pot full of coconut fiber.

Through lots of watering (dechlorinated, with a pressure sprayer) I spray off the plants, walls and rocks/gravel about every other day, washing waste particles down underneath the gravel where the nutrients can then feed the plants. I wasn't entirely sure how well this would work when I started, though considering how robust the plants are getting lately, it seems to be a success! I keep the water level below the gravel, enough to cover the roots, but not high enough to create surface puddles. The plants have a continuous supply of water, and the humidity inside stays relatively high to mimic a tropical environment.

Other details: the tank itself is a 15 gallon glass aquarium (same footprint as a standard 10 gallon, but taller) with a wire mesh lid. There are two lights on top, a 14 watt CFL on the left for plant light, and a 25 watt incandescent on the right for heat (hint: if you still can't find the gecko, he likes it where it's warmest!). They're on a timer, and on for 13 hours a day, similar again to light levels in the tropics. The plastic lid is a water dish, and the main hiding place is a broken ceramic bowl turned upside-down. ;)

Anyway, I just wanted to share this concept of growing plants in a gravel substrate, and that it does indeed work (if you have the right plants and maintain the water levels, anyway). I've seen a lot of terrariums set up using soil before, and my observation has generally been that they tend to deteriorate over time, as mold/etc. tends to take hold in the stagnant organic matter. Gravel is much easier to wash, and spaced widely enough to prevent too much stagnation - and I happen to think it looks nicer, too!