all the ones that have been mentioned, and i'd suggest platies as well. if you stock them, though, you should stick mainly to the primarily black/white breeds (such as panda platies) so the bright colors don't encourage attacking.
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)