So you see there, how her fin hangs down in a triangular shape, and his gonopodium is held up close to his body.
I'd have to disagree with allexx because swords get much too large for a 20 gal tank. The only livebearers I would stock in a 20 gal are guppies and platties.
There are many attractive platties out there. The other day at my local petco I became rather fond of the panda and bumble bee, and even easier to find is the red wag, sunset wag, and mickey mouse. Sometimes you will see HiFin versions as well.