I went to Bunnings today and picked up some of these for my office fish tank.
They're called water lettuce. Not surprising really, since they look just like floating heads of lettuce (but with long feathery roots that grow into the water). Being natives of the mighty Amazon, they also fit in well with the South American theme of my office aquarium. I just hope that my silver dollars (vegetarian relatives of the pirhana) leave the plants alone.
The tank, on the whole, is looking really good now....makes me want to settle into my office sofa, stare into the tank, and drink capaherinas all day.