Marimo

Marimo balls are not moss and not a plant — they are a spherical colony of filamentous algae (Aegagropila linnaei) that has been rolled into a ball by gential water movement in cold lakes of Japan, Iceland, and Scotland. The velvety green spheres are almost self-sufficient: they grow so slowly that a 10 cm ball may be decades old.

Low light and cool water are the only requirements. Temperatures above 25 °C cause the balls to brown and break apart over time — marimo is a cold-water organism and in tropical tanks it is a short-term novelty rather than a permanent fixture. A gentle roll by hand each week during water changes keeps the round shape and prevents the bottom from flattening against the glass.

When a ball grows too large, cut it in half and re-shape the pieces between your palms, though it takes months to restore a dense sphere. Pale spots indicate too much direct light — move the ball to a shadier spot. In nature the algae also grows as flat mats on rocks, so partial flattening in the aquarium is not a health concern.