Marimo (Cladophora aegagrophila)

Cladophora aegagrophila is not really a plant, but a ball of algae, so it is a decorative exception from the rule about avoiding algae at all costs. It is normally found in shallow lakes, where the movement of the waves forms it into a sphere. In an aquarium, it must be turned regularly to keep it in shape. Cladophora aegagrophila can be divided into smaller pieces, which become spherical with time, or which form a carpet if attached to roots and stones.