I purchased something at the pet shop called Fancy Duckweed. It really took a bit for me to find what this stuff actually was. I finally found that is is Spirodela polyrhiza. It grows like duckweed and multiplies like it but is much larger. There are a couple of regular size duckweed plants on the far left. The ruler is turned with the metric side toward the plants so those dimensions are in centimeters. The leaves of this plant are several times larger than regular duckweed (Lemna minor) although the specific smaller plant may be a bit different from the regular. It should have been easier to find since at one time the plant was known a Lemna major.