Quick answer: A marimo moss ball is not a moss at all. It is a slow-growing green algae, Aegagropila linnaei, that rolls itself into a velvety ball on the floor of cold lakes in Japan, Iceland and Northern Europe. Marimo are one of the easiest living things you can keep. Give them cool water around 64 to 77°F (18 to 25°C), low to moderate light out of direct sun, a water change every one to two weeks in a jar, and a gentle squeeze and roll now and then to keep them round. They grow only about 5 mm (0.2 inch) a year, so a ball the size of a marble is already several years old. The mistakes most people make are putting them in a sunny window, which turns them white or brown, and keeping them in very warm water, which slowly weakens them.
Marimo moss ball at a glance
| Detail | Marimo moss ball |
|---|---|
| Scientific name | Aegagropila linnaei (formerly Cladophora aegagropila) |
| What it is | A green filamentous algae, not a moss |
| Other names | Marimo, moss ball, Cladophora ball, lake ball, Japanese moss ball |
| Native to | Cold lakes in Japan, Iceland, Northern Europe and a few other northern regions |
| Growth rate | About 5 mm (0.2 in) of diameter per year |
| Size sold | Usually 1 to 5 cm (0.4 to 2 in) |
| Size in the wild | Up to 20 to 30 cm (8 to 12 in) in Lake Akan, Japan |
| Temperature | 64 to 77°F (18 to 25°C), happiest at the cooler end |
| pH | About 6.5 to 8.5 |
| Light | Low to moderate, indirect. No direct sunlight |
| Water changes | Every 1 to 2 weeks in a jar; normal aquarium changes in a tank |
| CO2 and fertilizer | Not needed |
| Lifespan | Many years, even decades, with stable care |
| Care level | Very easy |
Marimo vs java moss vs fake moss balls
Marimo are sold next to real aquarium mosses and, sometimes, cheap imitations. Here is how they compare.
| Feature | Marimo moss ball | Java moss | Fake or “wrapped” moss ball |
|---|---|---|---|
| What it is | Green algae | A true moss | Loose algae or moss wrapped around a core |
| Shape | Dense, round, holds shape on its own | Loose, stringy tangle | Round at first, often lumpy or falling apart |
| Texture | Velvety, soft all the way through | Fine, wiry strands | Hard center, often a stone, foam or plastic |
| Attaches to decor? | No, rolls free | Yes, with rhizoids | No |
| Growth | Very slow, about 5 mm a year | Moderate to fast | Varies |
| Best for | Jars, nano tanks, shrimp and snail tanks | Covering wood and rock, shrimp tanks | Not recommended |
Here is the part most guides leave out. A real marimo is soft all the way to the middle, with no hard core. If you gently squeeze a moss ball and feel a stone, plastic or foam center, it is a “made” ball, usually loose algae rolled around something to look like an old marimo. They are not dangerous, but they rarely hold their shape and often fall apart within months.
What is a marimo moss ball?
Marimo are a rare natural growth form of the green algae Aegagropila linnaei. In most places this algae grows as a loose mat or a fuzzy coating on rocks. In a handful of cold, shallow lakes, gentle waves roll the algae along the bottom, and over many years it forms dense, perfectly round balls. Each ball is made of thousands of tiny branching algae threads growing out from the center.
The name “marimo” is Japanese and roughly means “bouncing water plant” or “seaweed ball”. The most famous marimo come from Lake Akan on the island of Hokkaido, where giant balls up to 20 to 30 cm (8 to 12 inches) across grow on the lake floor. They are protected in Japan as a natural treasure and have been protected in Iceland’s Lake Mývatn too.
The balls sold for aquariums are not taken from those protected lakes. They are farmed, usually by hand-rolling clumps of the same algae and letting them grow, which is why most store marimo are small, from marble to golf ball size.
Check new marimo for zebra mussels
In 2021, invasive zebra mussels were found hidden inside marimo moss balls sold in pet stores across the United States, and the problem spread to dozens of states. Zebra mussels are tiny striped shellfish that can wreck lakes, pipes and water systems if they get into the wild.
What to look for. Before adding a new marimo, look it over closely and gently squeeze it. Zebra mussels look like small hard shells, about the size of a fingernail or smaller, with dark zigzag stripes. Baby mussels can be much smaller and feel like grit.
If you find any. US Fish and Wildlife Service advice is to destroy the moss ball and any mussels by freezing it in a sealed bag for at least 24 hours, boiling it for at least one minute, or soaking it in diluted bleach or undiluted white vinegar. Then seal it in a bag and put it in the trash.
Never flush a marimo, pour its water outside, or throw it into a pond, stream or storm drain. Buying from a trusted aquarium shop and rinsing new balls in a bucket before use are simple ways to stay safe.
How to set up a marimo jar or tank
Marimo do well in a simple glass jar, a vase, a nano tank or a full aquarium. They need no filter, heater, substrate or CO2 if kept on their own.
Container. Any clean glass container works. Give each ball enough room to sit on the bottom without being squashed. A jar of 0.5 to 1 liter (about 2 to 4 cups) suits one or two small balls. Larger containers stay cooler and more stable.
Water. Use dechlorinated tap water, or tap water left to stand for a day if your supply uses chlorine rather than chloramine. Bottled spring water also works. Avoid distilled water on its own, since it has no minerals at all.
Light. Bright, indirect light or a gentle aquarium light is ideal. In the wild, marimo grow at the bottom of lakes in low light. Direct sun cooks the water and bleaches the algae, so keep jars away from sunny windowsills.
Temperature. Keep the water around 64 to 77°F (18 to 25°C). Marimo can handle warmer water for a while, but long spells above about 80°F (27°C) stress them and can cause browning or rot. In hot weather, move the jar to the coolest spot in the room. Some keepers put the jar in the refrigerator for a few hours, never the freezer.
In an aquarium. Marimo fit well in planted tanks, shrimp tanks and nano tanks. They are a classic choice for a 5 gallon tank or a desk aquarium. Place them on the substrate where they get some light but are not under the brightest part of the lamp. Very warm tropical tanks at 80°F and above are not ideal for them long term.
Marimo water changes and cleaning
In a jar or vase. Change the water every one to two weeks, and more often in summer. Pour out the old water, rinse the jar, and refill with fresh, dechlorinated water at about the same temperature.
In an aquarium. Your normal weekly or every-other-week water changes are plenty.
Squeeze and roll. Every few weeks, take the marimo out and gently squeeze it in a bowl of clean water, like a soft sponge. Dirt and old water come out, and it soaks up fresh water. Then roll it gently between your palms to keep it round, and put it back.
Turn it. In the wild, waves keep marimo rolling so every side gets light. In a jar, give it a gentle turn every few days or at each water change. A side that always faces down can turn brown from lack of light.
Why do marimo moss balls float?
A floating marimo is usually a healthy one. Like plants, marimo make oxygen during photosynthesis. Tiny bubbles get trapped between the algae threads, and the ball becomes light enough to float. When the light goes off, the bubbles slowly dissolve and the ball sinks again.
New marimo often float for a day or two because air is trapped inside after shipping. Give it a gentle squeeze underwater to push the air out and it will sink.
A marimo that floats all the time, even in the dark, may have a pocket of trapped air or, rarely, gas from rot inside. Squeeze it gently underwater. If it smells bad or feels mushy in the middle, see the problems section below.
How big do marimo moss balls get?
Marimo grow very slowly, about 5 mm (0.2 inch) in diameter a year, and often less in warm or dim homes.
| Diameter | Rough age |
|---|---|
| 1 cm (0.4 in) | About 2 years |
| 2.5 cm (1 in) | About 5 years |
| 5 cm (2 in) | About 10 years |
| 10 cm (4 in) | About 20 years |
| 20 to 30 cm (8 to 12 in) | Many decades, only in the wild |
Farmed marimo may grow a little faster than wild ones because they are rolled into shape, so treat these ages as rough guides. Either way, do not expect visible growth in weeks. If your marimo looks the same size after a year, that is completely normal.
How to split and propagate marimo
Marimo reproduce by breaking into pieces, and you can do the same at home.
- Wait for a good-sized ball. At least 3 to 4 cm (about 1.5 inches) is easiest to split.
- Pull it apart gently. With clean hands, tear the ball into two or more pieces.
- Roll each piece into a ball. Squeeze out the water and roll each piece between your palms until it holds a round shape.
- Hold the shape if needed. Some keepers wrap new small balls loosely with cotton thread for a few weeks until they hold together. The cotton breaks down on its own.
- Keep them cool and lightly lit. Turn them often so they grow evenly.
Sometimes a marimo splits by itself or sprouts a small bump that later breaks off. That is natural and not a sign of trouble.
Marimo moss ball tank mates
Marimo are peaceful and safe with most small aquarium life. They also give shrimp and snails a surface to graze.
| Good with marimo | Avoid with marimo |
|---|---|
| Cherry shrimp and Amano shrimp | Goldfish, which tear them apart and eat them |
| Nerite snails and mystery snails | Large plecos and other big grazers that flatten or shred them |
| Bettas and small peaceful fish like tetras, rasboras and corydoras | Crayfish and large cichlids that dig and pull things apart |
| Otocinclus and small algae grazers | Very warm tanks, above about 80°F, long term |
Shrimp love marimo. They spend hours picking through the surface for biofilm and food bits, which also helps keep the ball clean. If you are planning a shrimp tank, a few marimo and some flame moss or java moss make a great low-maintenance base. For more easy plants, see our low light aquarium plants list.
Do marimo moss balls clean the water?
A little. Marimo take up nitrates and other dissolved nutrients as they grow, and they compete with other algae for those nutrients. But they grow so slowly that they will never replace water changes or a filter. Think of them as a small bonus, not a cleaning system. If your tank has an algae problem, the fix is usually light and nutrient balance, and our algae eater fish guide covers what actually eats each type of algae.
Common marimo moss ball problems
Turning brown. Usually not enough light on one side, dirty water or water that is too warm. Change the water, squeeze the ball clean, turn the brown side toward the light, and move it somewhere cooler. Small brown spots often recover green within a few weeks.
Turning white or pale. Too much light, usually direct sun. The algae is bleaching. Move it to a shadier spot right away. Pale patches can recover slowly, but fully white sections may not.
Black spots or a soft, smelly center. Rot, often from warm water or dirty water left too long. Take the marimo out, carefully remove the black or mushy parts, rinse it in cool clean water and roll the healthy part back into a ball. If the whole ball is mushy and smells bad, it is usually beyond saving.
Falling apart. A very loose or broken ball is often a sign of a fake or a “made” marimo, or of rough handling. Roll the healthy pieces into new small balls, as in the propagation steps above.
Fuzzy white growth on the ball. Usually a harmless fungus or biofilm feeding on dead bits. Squeeze and rinse the ball and remove any dead patches. Our white fuzz guide explains what causes it in tanks.
Other algae growing on it. Hair algae or other algae sometimes grows on a marimo in a bright tank. Pick it off by hand and lower the light. Amano shrimp are very good at keeping the surface clean.
The jar water looks cloudy. Change the water more often and rinse the jar. Cloudy water usually means the water has been left too long or the jar is too warm. Our cloudy water guide covers the causes in aquariums.
No growth at all. Normal. Marimo grow only a few millimeters a year. As long as the ball stays green and firm, it is healthy.
How long do marimo moss balls live?
Marimo can live for many years, and in the right conditions for decades. In Japan they are often described as heirlooms that can be passed down through a family, and the giant wild balls in Lake Akan take many decades to form. At home, a marimo kept cool, out of direct sun and in clean water should easily last 10 years or more. Because they keep growing from the outside and can be split and rerolled, a single marimo can, in theory, keep going indefinitely.
FAQ
Is a marimo moss ball a plant or an animal?
It is a living green algae, which is plant-like and makes its own food from light. It is not a moss and not an animal, even though it is often sold as a “moss ball pet”.
How often should I change marimo water?
Every one to two weeks in a jar or vase, and more often in hot weather. In an aquarium, your normal tank water changes are enough.
Can marimo moss balls live with a betta?
Yes. Marimo are safe with bettas and are a popular addition to betta tanks. Keep the tank at the lower end of the betta’s range if you can, around 76 to 78°F.
Why is my marimo moss ball floating?
Usually because oxygen bubbles from photosynthesis are trapped inside it. That is a sign it is healthy. A gentle squeeze underwater releases trapped air if you want it to sink.
Why is my marimo turning brown?
Most often from too little light on one side, dirty water or water that is too warm. Change the water, clean the ball, turn it regularly and keep it cool.
Do marimo moss balls need sunlight?
They need light, but not direct sun. Bright, indirect daylight or a normal aquarium light is best. Direct sunlight bleaches them white and overheats the water.
Do marimo moss balls need a filter?
No. A marimo kept on its own in a jar only needs regular water changes. In an aquarium with fish, the filter is for the fish, not the marimo.
Can you cut a marimo moss ball in half?
Yes. Gently pull it apart into two or more pieces and roll each piece into a ball. With time, each one grows into a new marimo.
Are marimo moss balls safe for shrimp?
Yes, they are one of the best additions to a shrimp tank. Shrimp graze biofilm on the surface all day and do not damage the ball.
How can you tell if a marimo is dead?
A dead marimo turns fully black, gray or white, falls apart easily and often smells bad. A marimo that is still green, even just on the outside, is alive and can recover.
Final word
Marimo moss balls are one of the easiest and most charming living things you can keep. Remember that they are a slow, cold-water algae, not a moss. Keep them cool, give them gentle indirect light, change their water every week or two, and squeeze and roll them every few weeks. Check new ones for hidden zebra mussels before use. Look after them like that and a small marimo can stay green and round for many years, slowly growing into a little piece of living history in your home.
