in a 20 gallon i have a few ideas.
escargot shells ( spelling error ) with some multies, you could even do a pair of BN plecos to in that tank,i wouldnt go over 8 to 10 max in that tank though, with about 100 shells.
or
livebearer tank with a trio of guppys, trio of swordtails, trio of mollys ( trio meaning one male, 2 females )
or
8 to 10 neons ( or smaller schooling tetras )
6 corydoras ( of your choice )
1 groumie ( spelling error, or an angel )