Duckweed will soon take over your tank surface. One day you have two itty, bitty leaves and the next an explosion!
I'm slowly eradicating mine as it gets in the cracks of the driftwood, deprives other plants of light and, worse, gets all over the internal filter. It is even trying to choke out my Red Root Floaters because I got lazy about removing it.
I posted a thread a while back on how to contain floating plants to one area of my tanks and got some really creative ideas.
I agree, it's really cute, though. ^smile^