Background Plants for a 20 Gallon High

I'm fond of Crypt crispatula (Ruffled Ribbon).

personally i like my hornwort and ancharis and also i think jungle vals or spiral vals would look good

Depending on their availability in your area, Isoetes japonica and Isoetes lacustris (commonly known as quillwort) both make excellent background plants in medium to tall tanks. They are easy to care for (requiring very little in the way of pruning), grow slowly even under medium to high light, do not need CO2 supplementation, have a striking bright green coloration, and put out very unique looking, rigidly vertical, tightly bunched shoots of needle-like leaves. I. japonica and I. lacustris look very nearly identical so either one would fit the bill.

I know that Robert at www.aquabotanic.com regularly carries I. lacustris and sells it for a very reasonable price.