Re: 12x12x18 - Help me visualize
That small of a viv can be tricky but just try and use broms towards the upper part and maybe 2 or three smallish plants towards the bottom. I personally like silver ribbon ferns but it may not look right in such a small viv. I also enjoy different types of pellonia,pilea, and peperomia.
One of my favorite pilea species is baby tears and after a while, it spreads and creates a nice carpet. Ficus pumila is very popular as it climbs the back of the viv and creates a shingle and dense green look on the background but if you don't trim it accordingly it will take over a 12x12x18. You may also want to look at ferns like a white rabbits foot fern. Personally, I stay away from begonias because while they can do really well I find they tend to melt away with the humidity.