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.

Here is an excellent and attractive addition to any freshwater tank. The Marimo ball, also known as Moss ball or Japanese moss ball is actually a filamentous green algae (Chlorophyta) found in a number of lakes in the northern hemisphere: Japan, United Kingdom, Russia, Iceland and Northern Europe.

Scientific name: Fissiden fontanus
Common names: Phoenix moss
Family: Fissidentaceae
Maximum size: Infinite
Light needed: Medium
Water conditions: 73.4-82.4°F (23 – 28 C), pH 6.0 – 7.5, water soft to hard
Hardiness: easy to moderate

Image by AquaticMagic
Scientific name: Vesicularia ferriei
Common names: Weeping moss
Family: Hypnaceae
Maximum size:
Light needed: from very low to high
Water contidions: 68 – 83 F (20 – 28 C), pH 6-8,
Hardiness: Very easy

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