I just saw Baensch Aquarium Atlas 3 (this is the English version from Tetra)
at my local pet store. It has a small (smaller than vol 1 and vol 2) section on
plants. One of the plants mentioned is called Sinking Riccia (Riccia rhenana).
The book says this species does root and sink in the water with good lighting.
If the lightin is poor, it will just float just under the water.
The origin section just says "Cosmopolitan." I really wonder what it means.
Could this be the species that Amano used in his tank ? I can't tell the
difference (from the picture) between this species and R. fluitans.
Does anybody have more info or has experience with this particular species of