There is a very nice small display tank in a fish store here, probably about 60/70 gal and square and shallow. It is mainly sand bed with minimal rock and only various gobies and scooters as fish, plus mainly fan worms as inverts. It is really beautiful. I'll try to take a pic next time I visit it. It has a wavemaker surge device in it which is very effective. Much bigger than you plan, but there is no reason I can see that something similar could not be achieved in 20 or 30 gal.