These fascinating little guys are not really plants at all, but spheres of dark green algae (even algae can be decorative!). They are normally found in shallow lakes, where the movement of the waves forms them into perfect spheres, and there they happily roll back and forth filtering nutrients from the water. Due to the action of photosynthesis, they rise to the surface during the day and sink back down at night. The Japanese government have actually designated them as a special national treasure and given them legal protection from harvesting.

In the tank they must be turned regularly to keep them in shape, but if you prefer they can instead be allowed to spread out and form a lush green carpet over the bottom. This looks especially lovely covering river rocks.

Kept in a spherical shape however, there are very few plants to rival this one for structural impact or interest in the aquarium. Best of all, despite their exotic appearance they are incredibly undemanding and easy to grow.

Summary of care:
Care Level: easy
Availability: all over ebay Cost: varies depending on availability but often quite expensive.
Max height: 10 cm
Max width: 10 cm
Growth: very slow
Goldfish Edibility Rating: not known but may be nibbled at, seeing as Golf Fish like algae. Snails do great with them.

Requirements:

Light level: very low to very high
Temperature: 5-28 C
pH: 6 - 8.5
Hardness: medium - very hard
Nutrients: not required
CO2: not required but does respond to added CO2
Water movement: does not mind water movement (will roll along the bottom if there is enough current), but do not place right underneath very strong flows as these may tear the sphere apart.
Planting: simply drop into the aquarium. Do not bury in the substrate.
Maintenance: none, except to turn over regularly if a perfect spherical shape is wanted.
Propagation: Divide original plant into smaller pieces and either roll these into spheres or allow to grow flat.