Well how big is your sorority in the 20 gallon? Second, do you have lots of plants/hiding spots? Any cleaner fish/cory's?
Check out my divided tank if you need some inspiration: http://www.bettafish.com/showthread.php?t=45372
Its not completed fully (as one betta died, and the left still needs some decor. :( but I recomend getting the Lee's Dividers, they have the big thick bumper and are easy to install. If your sorority is too small then they will fight, but if its a little bigger it will spread the aggression out.