Marimo Moss Ball (Aegagropila linnaei)

Marimo isn't a moss at all, despite the name everyone uses for it — it's actually a ball-shaped colony of Aegagropila linnaei, a filamentous green alga that grows into that spherical shape naturally in a handful of still and slow-moving lakes worldwide, most notably Lake Akan in Japan and Lake Mývatn in Iceland. In the wild, it rolls slowly along the lake bottom, rotating just enough that light reaches every side evenly, which is how it maintains the spherical shape rather than growing asymmetrically toward the light source like a typical algae cluster would.

In a tank, it's about as close to a completely undemanding freshwater plant-analog as the hobby offers: low light is fine, no substrate or fertilizer needed, and shrimp actively graze on the biofilm that accumulates on its surface, which makes it pull double duty as both décor and feeding station. It's one of the few aquarium organisms you can genuinely neglect and have it not just survive but look better over time as it continues to grow, if slowly — roughly 5mm of diameter per year under good conditions.

Give it an occasional gentle squeeze and rotation under water to clean out debris and maintain its spherical shape, and it'll sit happily in a tank for years or decades. The care is less aquarium husbandry and more basic object maintenance.

Scientific name: Aegagropila linnaei
Origin: Lake Akan (Japan), Lake Mývatn (Iceland), and a handful of other high-latitude cold and cool-temperate lakes
Water parameters: Temp 50–75°F (prefers cooler water; avoid sustained temps above 77°F) · pH 6.0–8.0 · Any hardness
Lighting: Low to moderate — direct high-intensity light causes browning; indirect or filtered light preferred
Tank requirements: Any size; works in nano tanks, shrimp tanks, and planted communities
Diet: Photosynthetic alga; no supplemental feeding needed
Husbandry note: If a marimo starts losing its spherical shape or developing brown patches, rotate it and ensure it's getting indirect low-intensity light from multiple angles. Brown patches are usually a light distribution issue, not a water chemistry problem.