Is a Marimo moss ball actually a plant?

No — despite the common name, it's a species of filamentous green algae, Aegagropila linnaei, that grows into a rare spherical form under the right wave conditions in a small number of lakes. It's not moss and not a vascular plant, but it's cared for and sold as an aquarium "plant" because that's functionally how it's used.

Are Marimo moss balls wild-harvested?

The famous wild marimo of Lake Akan in Japan are legally protected as a natural monument and are not sold commercially. Nearly all marimo sold in the aquarium trade are cultivated — loose strands of the same algae grown in aquaculture (much of it in Ukraine) and hand-rolled into balls, not taken from a protected wild population.

Does a Marimo moss ball need light or fertilizer?

Low to moderate light is enough — it doesn't want direct sun or intense aquarium lighting, which can bleach it. It needs no fertilizer or CO2 at all; it pulls what little it needs directly from the water column and ambient light.

Why is my Marimo moss ball floating instead of sinking?

Trapped gas bubbles from photosynthesis, usually because it hasn't been rotated or gently squeezed recently. Take it out, squeeze it gently underwater to release trapped air, and return it — it should sink again.

How big do Marimo moss balls get?

Very slowly — roughly 5mm of growth per year. Most sold for aquariums are 1-2 inches across; reaching a larger size takes many years, which is part of why undisturbed wild specimens can be genuinely old.

How do you maintain a Marimo moss ball?

Turn or roll it every week or two so it grows evenly on all sides rather than developing a dark, flattened spot from resting in one position. Every month or so, take it out and gently squeeze it in tank water to flush out trapped detritus.

Marimo Moss Ball (Aegagropila linnaei): The Plant That Isn't a Plant

Marimo moss balls are one of the strangest success stories in the aquarium hobby: something sold and cared for exactly like a plant that, taxonomically, isn’t one. Aegagropila linnaei is a species of filamentous green algae that, under specific wave conditions in a handful of lakes worldwide — most famously Lake Akan in Japan — grows into dense, velvety spheres instead of the loose mats or strands algae normally forms.

Not Moss, and Not Wild-Harvested

Two common misconceptions are worth clearing up. First, despite the name, it isn’t moss at all — true mosses are land or aquatic plants; marimo is algae, more closely related to the nuisance filamentous algae in the algae identification series than to Java Moss. Second, the marimo sold in stores almost never comes from the protected wild population — Lake Akan’s marimo are a legally protected natural monument in Japan and aren’t harvested for sale. Nearly all aquarium marimo are cultivated from the same algae species grown in aquaculture, then shaped and rolled by hand.

Why It’s So Easy

Marimo tolerates low light, doesn’t need CO2, and takes no fertilizer at all — there’s genuinely nothing to feed it. It’s slow-growing (roughly 5mm a year), which means it also doesn’t need trimming or thinning the way almost every other plant on this site does. It’s about as close to a truly maintenance-free living addition to a tank as exists in the hobby.

Water Parameters

It’s tolerant of a wide range: 60–78°F (16–26°C) and a broad pH range. It technically prefers cooler water than a typical heated tropical tank, but it’s kept successfully in standard community and betta tanks at normal temperatures all the time — this isn’t a strict requirement the way it is for some cold-water species.

Care and Placement

Keep it out of direct, intense light — unlike most plants on this site, too much light can bleach or damage marimo rather than helping it. A shaded spot or moderate ambient tank lighting is ideal. It works anywhere: resting on the substrate, tucked among hardscape, or simply free-floating in a nano or betta tank, and it’s a popular addition to shrimp tanks since it provides grazing surface without demanding anything back.

Maintenance

The one thing marimo genuinely needs: rotation. In the wild, wave action rolls it evenly so it grows as a symmetric sphere; in a static aquarium, a ball left in one position can develop a flattened, darker patch on the side resting against the substrate from lack of light. Turn or roll it every week or two to keep growth even. Every few weeks, take it out and gently squeeze it in a bowl of tank water to flush out trapped detritus and debris that accumulates in the strands — this also helps it keep its shape.

Common Problems

Floating instead of sinking. Trapped gas from photosynthesis — squeeze it gently underwater to release the bubbles and it will sink again.
Browning or a dull, faded patch. Usually either too much direct light (bleaching) or a flattened side from not being rotated — address both.
Falling apart into loose strands. Typically from rough handling or repeated hard squeezing; handle gently and it holds together well under normal care.

Pests

No pest issues of its own. As with any new addition, a quick rinse before adding it to an established tank is good practice.

Marimo is a rare case in this hobby: a genuinely engaging, visually distinctive addition that asks for almost nothing in return. If you want a living centerpiece for a nano or betta tank without adding another thing to your fertilizing routine, it’s hard to beat.