The small Serpae Tetra has been one of my personal favorites since I was a kid, possibly because their red, white and black colors are the same as my beloved soccer team :-)

They're quite peaceful towards other species, but spend a lot of time disputing ranks among themselves. It never seems to reach the point of actual aggression and injury, though. During these disputes, they open their fins real wide and dance around each other like boxers, occasionally taking a charge towards the opponent. Their beautiful colors are also boosted to their best, far more intense than what's shown in the picture above. I'm still hoping to capture one of these disputes in a photograph, but it's hard because when I approach they stop and turn their attention to me.