a little advise to the new 20 gal. Take all the advice you can get. I started my 20 in Feb. and the first lesson i learned was not to get in a hurry about adding stock. it takes a long time. I started out with the sand,gravel and live rock. after a couple weks i got the clean up crew, i should have got the 2 percula clowns and stopped.
but i got in a hurry and got anemones (that have all died) shrimp (died) and my freakin blue legged hermit crabs are eating my snails.
So ask questions on this board before you take steps, If i had done that i'd have not been out the money nor would the things i bought died.
good luck.