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Marimo Moss Ball
Aegagropila linnaei
Care level: Easy
Found rolling gently along the floor of a small number of cold lakes in Japan, Iceland, and Northern Europe, Marimo forms slow-growing spherical colonies that can live for over a century and take decades to reach the size of a grapefruit. Water currents in these lakes tumble the algae mass evenly on all sides, which is what keeps it growing into a neat, symmetrical ball rather than a flat mat.
Standard round MarimoMarimo moss "carpet" (unrolled)
Light
Low to moderate aquarium lighting (~10-30 PAR) is ideal; too much light or warmth can cause browning and slow decline.
Water
Weekly 20-30% water changes; prefers cooler water than most tropical setups, pH 6.5-7.5, 18-24°C.
CO2 & Feeding
No CO2 or fertilization needed at all, it grows extremely slowly and draws minimal nutrients from the water.
Substrate
No substrate used and it should never be planted, it simply rests loose on the substrate or is left to roll and drift with gentle current.
Temperature
18-24°C. It prefers cooler water than most tropical tanks.
Growth speed
Slow
Tank Placement
Midground
Did you know?
Marimo is not a true plant at all, it's a filamentous green algae species that forms itself into a dense ball, making it one of the very few "aquarium plants" in the entire hobby that is actually classified as algae.
Care by Season
Spring
Gently squeeze out and rinse in tank water to remove trapped debris as part of routine maintenance.
Summer
Watch for excess heat, which can stress it; keep it out of direct light or warm equipment runoff.
Autumn
Continue routine turning and rinsing; growth is essentially imperceptible season to season.
Winter
Tolerates cooler water better than most tropical plants, making it a good match for unheated nano tanks.
Growing & Maintenance
Planting
Do not plant or bury it. Let it rest loose on the substrate, roll it over regularly and squeeze it out gently in tank water now and then.
Fertilising
None needed, and no CO2 either. It draws very little from the water.
Pests & Diseases
Common problems
Brown patches
Most likely cause: Too much light or heat on one side, or trapped debris.
What to do: Roll the ball over regularly, rinse it and move it out of strong light.
Brown underside
Most likely cause: The ball is never turned, so one side gets no light.
What to do: Turn it at every water change.
The ball falling apart
Most likely cause: Too much warmth, or debris inside.
What to do: Rinse it gently, keep it cooler and press it back into shape.
Strands of hair algae on the ball
Most likely cause: Algae attaching to the surface.
What to do: Pick the strands off by hand, or squeeze and rinse the ball.
No visible growth
Most likely cause: Normal. Its growth is almost imperceptible.
What to do: Nothing to do.
Pests & Diseases
Browning or discoloration
Signs: Outer surface turns brown or dull, especially on the side facing the light
Treatment: Roll the ball over regularly and gently squeeze it out in tank water to release trapped debris and mulm.
Hair algae attachment
Signs: Fine green threads of a different, unwanted algae growing on the surface
Treatment: Manually pick off strands or gently squeeze and rinse the ball; improve overall tank nutrient balance to discourage recurrence.
FAQ
Why is my Marimo ball turning brown or falling apart?
This usually means it's not being turned regularly and is getting too much light or heat on one side, or debris has built up inside; gently squeeze it out in tank water and roll it over regularly to keep it round and green.
Does Marimo need to be planted in substrate?
No, it should never be planted or buried; it simply sits loose on the substrate or floats, exactly as it does rolling freely along a lake floor in the wild.
How much light does a Marimo need?
Low to moderate light. Too much light or warmth turns it brown.
What water temperature does a Marimo need?
Between 18 and 24°C. It prefers cooler water than most tropical tanks, which makes it a good match for an unheated nano tank.
How do I clean a Marimo ball?
Squeeze it out gently and rinse it in tank water. That removes the debris trapped inside.
Is a Marimo a moss?
No. It is a filamentous green alga that grows into a dense ball.
