male betta(1 only)
or some male guppies(no females with males unless you want alot of babys)
o and with the betta you could put the golden mystery snail
her are some links
golden mystery snail -----------
http://farm2.static.flickr.com/1137/531117268_19c85ae888.jpg?v=0
http://www.aquanourriplus.com/boutique/images/apple_snail.jpg
http://www.microscopy-uk.org.uk/mag/imgnov06macro/cm/snail11.jpg
betta---------------
http://animal-world.com/encyclo/fresh/anabantoids/images/Betta(bluemale)WFA_Ap4Be.jpg
http://www.utahadventurevideos.com/images/betta_fish.jpg
http://www.jotyabetta.com/image/STOCK%20SHOP/ct008.jpg
guppie---------------
http://www.guppy-fish.com/guppy/img/tuxedo/image/tuxedo03.jpg
http://www.aqua-fish.net/imgs/fish/guppy6.jpg
http://www.breeding-guppies.com/images/guppy2.jpg