Marimo Moss Ball (Aegagropila linnaei)

Choose a marimo moss ball for low light and cool water

The Marimo Moss Ball (Aegagropila linnaei) is not a moss at all but a filamentous green alga that grows into a soft, velvety, bright green ball. It needs no planting, no CO2 and very little light, which makes it one of the easiest living additions to a shrimp tank, a nano aquarium or a coldwater set-up.

Marimo belongs to the green algae order Cladophorales, in the family Pithophoraceae. You will also see it sold as a moss ball, Cladophora ball, lake ball or Japanese moss ball. Wikipedia notes that some recent sources give the name as Aegagropila brownii; the aquarium trade and the recent research cited here still use Aegagropila linnaei. The Japanese name was coined in 1898: “mari” is a bouncing play ball and “mo” a general word for water plants.

In the wild, marimo grows in three forms: on rocks, as loose filaments on lake beds, and, in a few lakes, as free-rolling balls kept round by gentle wave action. The famous balls of Lake Akan in Japan reach 20–30 cm and have been protected since the 1920s; Lake Mývatn in Iceland held colonies of balls about 12 cm across at 2–2.5 m deep. Collecting from Lake Akan is prohibited, so aquarium balls are cultivated rather than taken from those lakes.

Check the fit before you order

New to live plants? Our live plants for beginners guide covers the basics. The photograph shows the type of ball you will receive, not the exact ball.

Choose your size

One purchase supplies one marimo moss ball in the size you choose: small, about 1–2 cm across, or medium, about 3–4 cm. Each ball is a single living mass of green filaments radiating from the centre, green all the way round.

Buy the size you want to see, because marimo grows very slowly. Wikipedia and Aquarium Co-Op give about 5 mm a year. A 2023 study of giant marimo in Lake Akan by Nakayama and colleagues estimated up to about 11.5 mm a year in the lake, and cited cultivation experiments of about 4.0 mm a year at 10°C and 6.0 mm a year at 14°C. In a home aquarium, expect a small ball to stay small for years.

Pick a size for your tank

Several small balls grouped together give a softer, more natural look than one larger ball. Marimo is sent with our livestock; see our delivery information for dispatch days.

Place it in low to medium light

Marimo is adapted to low light. Wikipedia describes wild colonies depending on that adaptation, with balls green on every side so any side can photosynthesise, and inner chloroplasts that stay dormant but switch on within hours if a ball breaks open. Aquarium Co-Op advises low to medium light and keeping balls out of direct sunlight. Flowgrow lists a wide light range, low to high, but strong light is the most common reason keepers give for a ball changing colour.

Pick a spot by light level

Turn the ball now and then so every side gets some light. In the wild, wave action rolls the balls and keeps them round; in an aquarium a gentle turn every week or two does the same job.

Keep the water cool to moderate

Marimo comes from cool northern lakes, and temperature matters more to it than light or nutrients. Flowgrow gives a tolerance of 4–28°C with best growth at 15–24°C, pH 6–8.5 and carbonate hardness 2–18 dKH. The Lake Akan study cites a laboratory experiment in which loose marimo filaments grew best at 22°C, the warmest temperature tested in a 5–22°C range, and another in which balls held at 35°C for 24 hours fell apart after 80 days.

Read the values side by side

That makes marimo ideal for coldwater and unheated tanks, and fine in most tropical tanks kept at the cooler end. Growth slows as the water warms, so place it away from the heater. Check your tap water with our water matcher if you are matching it to particular fish or shrimp.

Clean, turn and reshape it

Marimo needs very little care, and most of it fits into your normal water change. Aquarium Co-Op suggests rinsing a new ball in aquarium water before it goes in, letting shrimp and snails pick off leftover food, and gently rolling the ball in your hands at each water change to help it keep its shape.

Follow a simple routine

1. On arrival, rinse the ball in a bowl of water taken from your aquarium, never under a hot tap.
2. Look it over for snails, shells or debris, and remove anything you find.
3. Place it on the substrate; if it floats, give it a gentle squeeze under water.
4. At each water change, lift it out, swish it in removed tank water and roll it gently between your palms.
5. Pick off any long strands of other algae growing on it.
6. Turn it so a different side faces up, then put it back.

A new ball may float at first, Aquarium Co-Op notes, and sinks once it is waterlogged. A ball that floats after weeks in the tank usually has gas trapped in the filaments, often after strong light; a gentle squeeze releases it.

Split it or spread it

Because marimo grows so slowly, most keepers make more balls by dividing one rather than waiting. Flowgrow lists splitting as the way it propagates. Aquarium Co-Op describes the method: squeeze the water out, cut the ball in half, roll each half into a sphere and tie it with cotton sewing thread so it holds its shape while it regrows. Some keepers unroll balls into flat mats instead, to cover stones or the floor of a small tank.

Divide a ball step by step

• Choose a healthy ball: green throughout, with no soft or brown core.
• Work wet: keep the pieces in tank water so they do not dry out.
• Roll firmly: shape each piece into a tight ball between your palms.
• Tie loosely: wrap cotton thread around it; the thread can stay and the filaments grow over it.
• Be patient: the new balls take months to look smooth again.

Check every new moss ball for hitchhikers

Like any live plant, a moss ball can carry small animals into your tank. The best-known example came from the United States: in February 2021 an employee of a Seattle pet shop found zebra mussels attached to and inside marimo balls sold as aquarium plants, and by late April 2021 contaminated moss balls had been found in 46 states, according to the US National Invasive Species Information Center. Owners there were urged to destroy suspect balls rather than dump them.

That episode is the reason for a quick check with any living plant from any source. Look the ball over in a bowl of tank water, feel for anything hard, and remove snails or shells before it goes into your aquarium.

Dispose of unwanted balls safely

• Never put a moss ball, its water or its packaging into a pond, river, drain or toilet.
• To dispose of a ball, kill it first; Wikipedia records advice to boil, freeze or bleach moss balls before disposal.
• Put the treated ball in household waste.
• OATA reminds keepers that releasing aquarium animals or plants into the wild is illegal.

Troubleshoot colour and shape

A healthy marimo is an even, deep green. Changes in colour or shape are slow to appear and slow to recover, so act on them early.

Pair each problem with an action

The same habits that keep marimo green keep the rest of the tank in balance. Our algae control guide explains how light and nutrients affect unwanted algae.

Pair marimo with gentle tank mates

Marimo suits peaceful tanks best. Shrimp and snails graze the surface and help keep it clean, and small, calm fish leave it alone. It is a natural partner for a cherry shrimp tank, and our shrimp-keeping guide explains how to set one up.

Choose companions that leave it intact

If a fish starts tearing at the ball, move the ball to another tank rather than replacing it; a damaged ball can be re-rolled and will recover slowly in calmer company.

Answer marimo moss ball buying questions

Is a marimo moss ball really a moss?

No. It is a filamentous green alga (Aegagropila linnaei, family Pithophoraceae) that grows into a ball shape. The name “moss ball” comes from its soft, velvety look.

Which size should I choose?

Small 1–2 cm suits nano and shrimp tanks; medium 3–4 cm makes a clearer feature at the front of a larger tank. Marimo grows only a few millimetres a year, so buy the size you want to see.

How fast does a moss ball grow?

Very slowly: about 5 mm a year according to general sources. A 2023 study cites 4–6 mm a year in culture at 10–14°C, and up to about 11.5 mm a year for giant balls in Lake Akan.

Why is my moss ball turning brown or pale?

Usually too much light, water that is too warm or a ball that is never turned. Move it out of direct light and away from heaters, and roll it gently so every side gets some light.

Can marimo live in a tropical aquarium?

Yes, at the cooler end. Flowgrow gives a best range of 15–24°C and tolerance up to 28°C, and keepers have grown them in betta tanks at up to 80°F (about 27°C). Growth slows in warm water.

Why is my moss ball floating?

Air trapped in the filaments, often when it is new or after strong light. It usually sinks once it is waterlogged; a gentle squeeze under water releases the air.

Do moss balls need fertiliser or CO2?

No. Normal aquarium water is enough. Some keepers add fertiliser or CO2 to speed growth, but it is not needed.

How do I check a moss ball for hitchhikers?

Rinse it in tank water and look it over for snails, small shells or anything hard before it goes in. In 2021 zebra mussels were found in moss balls sold in the United States, so a careful check is worth the minute it takes.

Consult the references

• Wikipedia: Marimo — names, classification, growth forms, wild lakes, protection, growth rate and the 2021 zebra mussel finding.
• Nakayama et al. (2023), Scientific Reports — temperature and growth of giant marimo in Lake Akan, with cited culture and heat-stress experiments.
• Flowgrow aquatic plant database — temperature, pH, hardness, light and propagation.
• Aquarium Co-Op: Marimo moss ball care — light, rinsing, rolling, floating and splitting.
• National Invasive Species Information Center (USDA) — zebra mussels found in moss balls sold in the United States in 2021.
• OATA care guidance — never release aquarium animals or plants into the wild.