Yes, there are 20 gal. 'uprights' (they're called 'talls'), and I do think that that would be a good size for a first terrarium. If you can afford it though, go a little bigger, like a 30 gal tall. A false bottom is basically a fake bottom. That is, it appears to be the bottom, but there is really an air (water) space under it. You build them by cutting some sections of 2" (or so) PVC pipe, apparently the white PVC is better than the black for terrariums. (Something about the different plastics they're made of, I can't remember). Anyways, you cut the PVC sections tall enough so that you would have about an inch of water over your pump for your waterfall. Then you get some eggcrate (white or black plastic grid thing, often used as a diffuser in light fixtures) and cut it to the shape and size of the bottom of your tank, and you set that on top of the PVC pipes (which are siliconed to the bottom tank glass in an upright position), and you cut the neccessary holes for your pump tubing and cord. Voila! A false bottom. You can do a search on this forum for false bottoms if you don't think I explained it well enough for you. Someone else describing it differently might do the trick. So you don't end up needing a pond liner of any kind. Just have some rocks sitting on the false bottom under your waterfall. They will give the illusion of a pool, but all of the water will find it's way back to the false bottom and be re-circulated. As for woodsy plants, nothing better than ferns and moss! I also like to add interesting rocks from my collection and/or bits of driftwood or sea shells. Anything goes, really, as long as it's non-toxic, because it's your tank! You could put a little fairy figurine in there if you wanted to. Don't use potting soil for the substrate, unless you're using a really high quality one with lots of large chunks for better aeration. Orchid bark or bark mulch is a better choice, or even just pure vermiculite or perlite. I like to use bark because it looks more natural. Oh yeah, another good plant would be a miniature vine, such as Ficus pumila (standard and variegated forms) or Ficus quercifolia (less invasive and looks like mini english ivy). Make sure you post pics when you're done! =)

Thanks so much! You've made it much easier to understand. When you speak of the substrate, I'm assuming you mean the "soil". I should NOT use any kind of potting mix? The ferns and moss are exactly what I was considering using. I just like the idea of playing around with a miniture garden since I can't play outside now!
Thanks again for your suggestions!
Becky

I think the reason she said not to use it (one reason anyway) is that it can fall through the eggcrate into the water below. I think you COULD use it if you provided a layer between the media and the grating to prevent it falling through. It could be just about anything. Lots of people recommend plastic screen material. I would think that regular ol' landscape fabric used for weed barriers would work pretty well.

You could always just use individual pots of whatever media and 'hide' them with bark mulch or whatever. Really the possibilities are endless and there are no rules. And best of all, you've found one of the best places to ask questions already! Definitely do a search of some of the old threads where you will find a TON of ideas and tips and suggestions...but beware, you'll soon be dreaming of a giant tank the size of a dorm room. :)

Yes, I forgot to mention to put a layer of fibreglass screening or other such material over the eggcrate to prevent the substrate from falling though. I don't recommend soil though because in my experience it is far too easy to over-water, stays moist exceedingly long, and is not well-aerated. If your substrate has enough 'air bubbles', it can be soaking wet, and the plant's roots won't rot or suffocate. That's why I prefer bark. It's a more open mix. But lots of people do use soil, and I did at first too, so if you want to, go for it! I'd add some extra perlite and horticultural sand (NOT play sand!) to it though, to make it more open and airy, and don't pack it down when you're planting the tank. Just sprinkle the dirt in, and only firm it around the plants as neccessary (to keep them from falling over). This will preserve any air pockets. And yes, the main reason I keep houseplants is to keep me happy through the winter! (Half the year up here).

Here is a step-by-step I made when putting mine together. SA is right: you don't need a pond liner, unless you want a stream or a place for standing water aboveground. That is why I used the pond liner.

Here is a link that might be useful: Building a False Bottom and Waterfall