there are a lot of options....a betta sorority, a 4-way divided male betta tank, a community tank (livebearers are great for that), two or three goldfish but be warned they'll outgrow it! a school of guppies if you know someone who will buy or adopt a large number of babies. hell, you could even build up one side with a slope of substrate and make it a fish/amphibian or fish/reptile environment. the possibilities are endless!
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)