The goal looks phenominal!!!!!
What are your stocking plans? I think it would look coolest with exclusively bottom dwellers, really tiny ones. In this case I would suggest GAMMARUS (scuds) and/or cherry/ Crystal red shrimp. I know 18 L isn't microscopic, but the little critters would look like mountain goats on a mountain that way. If you added some other kind of plant it would look good with fishes too. (in my opinion)
PS: gammarus are not common in aquaria, they can be caught in most bodies of water, especially fishless ones. They eat detrius and decaying matter of plant or animal orgin, and algae. I kept them for a time and they are fairly hardy. I eventually fed them to my fishes because some people didn't like the idea of sharing a house with a bucket of "bugs and gunk." I am going to try again this spring using a pickle jar and live plants.
Bear in mind they used flame/stringy moss (or something similar.. I am no expert at moss IDing) in the corners and weeping moss (plus a couple other kinds) for the middle. I presume IL is illenois so you live in USA? If you do, go to aquabid and bid for some moss!
The LFS will probably have at least one variety. Look around town for deals and find what species are avaliable.
PSS you don't need a heater and moss does better without one. I have researched mosses a lot... although I haven't achieved anything worthy of even photographing. I have 1 of the following types of moss:
There is too little to tell but it is growing slightly erect... IE straight up. I am not into microscope photography or DNA testing...
ANYWAYS just good luck, and have fun! Get as many unique varieties as possible. Bear in mind star moss isn't truly aquatic.