Ooh I love this tank! The underwater section looks spectacular, the vals are especially awesome. I like the plant choice in the top part, but you might wanna spread around more bacopa stems to fill up the space.
I had an 18gal tall riparium with similar dimensions (20x10x20) and the same filter, but my plants outgrew it. Man, ripariums are notoriously difficult to photograph! (unless your name is Hydrophyte)