Your tank is 21"L x 11"W x 18" H. Once you manage to keep nitrates and phosphates as close to zero as possible, you could consider those corals that thrive in low light or even shadowed areas as long as you can meet their other needs (e.g. current strength, sand substrate instead of
live rock).
Here is a list of Soft Corals that might not be too delicate to keep, please consider their potential size as limiting factor (taken from Julian Sprung, 1999, Corals A Quick Reference Guide, Oceanographic Series, Ricordea Publishing, Florida).
Studeriotes (Christmas tree, French Tickler, Sphaerella krempfi): Quite hardy. Peaceful. Requires moderate to strong currents, best placed in the sand substrate (not live rock).
Minabea: Requires moderate to high currents. Considered peaceful. Quite hardy. Needs to be placed in a shadowed area.
Nephthyigorgia (Chili Sponge, Red Finger (soft) Coral, Devil's Hand): Peaceful. Placed at shadowed areas or at the lower third of the tank. Considered hardy.
Diodogorgia (Finger Sea Fan, Yellow Finger, Red Finger). Somewhat hardy. Peaceful. Needs moderate to strong currents. Placed at the lower end of the tank.
Guaigorgia (Blue Gorgorian, Sea Whip). Mild to Strong currents. Peaceful. Placed at the bottom of the tank (on live rock). Somewhat hardy.
Pepetj
Santo Domingo