Lemna minor - Wikipedia, the free encyclopedia
This. I always add some (I breed it for this purpose; got it from a petshop, so it isn't from a natural pond or something) when I change the water. I like how it looks (normally it covers whole surface of water, but Izy pulled out some when he was shedding). One day the plant from behind will cover whole root, so I'm hoping leaves will create some kind of roof above the water dish covered with duckweed.