Under good growing conditions, duckweed is an extremely fast growing plant that can double the area it covers in a day or two. I believe it is a really nice plant but it blocks the light, making other species hard to grow and I hate to have duckweek drip off the elbows everytime I stick my arms in the tank.

Scientific name: Riccia sp “Dwarf”
Common names: Crystalwort or Riccia Dwarf
Family:Ricciaceae
Maximum size: about 0.03 by 0.3 inch.
Light needed: Medium to High
Water contidions: pH 6.2 to 7, temp. 23 to 28, soft to hard water
Hardiness: Easy

Scientific name: Riccia Fluitans
Common names: Crystalwort or Riccia
Family:Ricciaceae
Maximum size: about 0.04 by 0.5 inch.
Light needed: Medium to High
Water contidions: pH 5.5 to 7.5, temp. 20 to 30, soft to hard water
Hardiness: Easy