Why has my Marimo moss ball started floating?

Floating is completely normal. During periods of high light, the Marimo photosynthesises rapidly, producing oxygen bubbles that get trapped in its fine filaments. This makes the ball buoyant. It will naturally sink again once the light dims and the oxygen dissipates, or you can give it a gentle squeeze to release the trapped air.

Why is my Marimo turning brown or white?

Brown or white patches usually indicate 'sunburn' from exposure to light that is too intense. Move your Marimo to a shadier part of the tank away from direct lighting or windows. Gently rotating the ball ensures all sides get even, low-level light and can help the discoloured patches recover over time.

How do I clean my Marimo?

Over time, your Marimo acts like a sponge, trapping fish waste and debris. During your regular water change, take the ball out and place it in a container of the siphoned tank water. Give it several gentle squeezes to expel the brown, murky dirt, then roll it lightly between your palms to restore its round shape before returning it to the tank.

Is the Marimo actually a type of moss?

No. Despite the common name, Marimo is entirely composed of a species of filamentous green algae (Aegagropila linnaei). It grows radially outwards from the centre, and the gentle tumbling motion of the water creates its unique, moss-like spherical appearance.

Can I cut my Marimo in half?

Yes, you can safely propagate Marimo by dividing it. Use clean scissors to cut the ball into halves or quarters. You can then gently roll the new pieces in your hands to make them round again, or you can flatten the pieces out and tie them to driftwood or rocks to create a dense, velvety carpet.

FreshwaterPlantsBeginner

Marimo Moss Ball

Aegagropila linnaei

The Marimo Moss Ball is a highly popular, exceptionally hardy aquatic organism that is, despite its common name, a unique spherical form of green algae.

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Ideal Conditions

Temperature: 5 – 25 °C
pH range: 6 – 8.5
Hardness: 2 – 15 dGH
Adult size: 2 – 15 cm
Suitable for nano aquariums and small unheated displays: 1 litres+
Lifespan: 10 – 100 years
Diet: Photosynthetic
Water flow: Low to moderate
Preferred setup: Place directly onto the substrate or attach to hardscape; avoid intense direct light

About the Marimo Moss Ball

The Marimo Moss Ball is one of the most distinctive additions you can make to a freshwater aquarium. Despite being universally known as a 'moss' ball, it is actually a colony of filamentous green algae (*Aegagropila linnaei*) that grows outwards from the centre. In nature, gentle wave currents roll the algae along the lake bed, ensuring all sides receive sunlight and shaping it into a perfect sphere. In the hobby, these dense, velvety green orbs are treasured for their unique appearance and incredibly slow growth rate. They are a staple of nano tanks and shrimp enclosures due to their ease of care.

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Where Marimo Moss Ball comes from

Native range

Northern Hemisphere lakes, East Asia, Northern Europe

Countries / regions

Japan, Iceland, United Kingdom, Estonia, Russia

Habitat

Shallow, cool freshwater lakes with gentle wave action that continuously rolls the algae to maintain its spherical shape.

Populations have declined significantly in places like Lake Mývatn due to nutrient pollution blocking sunlight.

Similar species

Java Moss
Taxiphyllum barbieri
Crystalwort
Riccia fluitans
Süßwassertang
Lomariopsis lineata

We’re adding more full species guides as the catalogue grows.

Compatible tank mates

Reviewed guidance based on this species' recorded traits.

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Cherry Shrimp
Neocaridina davidi
Dwarf shrimp absolutely love grazing on the biofilm and trapped detritus on the surface of the Marimo without consuming the algae itself.
Great
Siamese Fighting Fish
Betta splendens
Bettas frequently rest upon the soft, velvety surface of the moss ball, and they thrive in similar low-flow environments.
Great
Goldfish
Carassius auratus
Goldfish are notorious plant-eaters and foragers. They may pick at the Marimo ball, eventually tearing it apart and ingesting the filaments.
Compatible with caution
Common Yabby
Cherax destructor
Large crayfish will readily dismantle and consume the Marimo, using their claws to tear the delicate spherical structure to shreds.
Not recommended
Silver Dollar
Metynnis argenteus
Herbivorous fish like Silver Dollars view almost all plant and soft algae matter as a primary food source and will quickly decimate the ball.
Not recommended

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