20 gallons can be a great size. You've got some room for some interesting livestock, and the bulk of the work regarding water quality can be done with easily manageable water changes.
As Matt indicated, keep your eye on this forum section:
http://www.canreef.com/vbulletin/forumdisplay.php?f=233
The gloves are coming off and a few of us are doing "nano battle." You might get some good ideas.
