Marimo Moss Ball: Betta Fish, Benefits & Care


Common Names: Marimo, Mossimo, Moss Ball, Seaweed Ball, Algae Ball, Japanese Moss Ball, Lake Ball, Marimo Moss Ball
Scientific Name: Aegagropila Linnaeii
Growth Rate: 5 mm (0.2 inches) per year
Life Span: Over 100 years!
Reproduction: Propagation is most common

A marimo moss ball is a filamentous green algae that’s naturally found in Iceland, Japan, Scotland, Estonia and most recently, Australia. Despite some of their common names among aquarists, they are not moss at all. They do not affix themselves to anything like other species of moss. Moss balls were first discovered in the 1820s and are even a protected species in Iceland (Lake Myvatn) and Japan (Lake Akan). Lake Akan is home to some of the largest marimo on the planet, reaching up to 30 cm (11.8 inches).

Moss balls are solid spherical balls of dense algae and do not have a solid core to them. They create their own food through photosynthesis and are extremely easy to care for. Best of all, moss balls will not turn your tank’s water green like other forms of unwanted algae. With that said, make sure you don’t add any algae killing chemicals to your aquarium. There are lots of benefits to keeping marimo as a tank mate, especially since they don’t produce any waste.

Marimo Moss Ball for Betta Fish

Moss Balls for Betta Fish

Betta fish and moss balls can live in perfect harmony. Aside from their natural velvety-green splendor and contrast amongst other decor, betta fish love playing with and resting on marimo moss balls. One minute it’s a velvety couch and the next it’s a soccer ball to push around. Ghost shrimp and aquarium snails, other common betta tank mates, are also big fans of the moss ball because of its ability to harbor small particles of food for feasting.

Marimo moss balls can live in the same water parameters that bettas thrive in, however they do grow faster in colder temperatures. When they are fully saturated with aquarium water they will sink to the bottom and only move from being disturbed. Moss balls also work to retain good tank bacteria, emit oxygen, and reduce nitrate levels which can stress betta fish health at high levels.

Marimo Moss Ball Care

Caring for your moss ball is very straightforward under normal conditions and health. They are basically indestructible and will only need occasional maintenance. In clean water and if cared for properly they could live longer than you.


Wondering whether moss balls need extra fertilizer or food? Nope, they make their own through photosynthesis using natural or artificial light.


Moss balls are cool because they’re round living balls of algae right? If a marimo moss ball stays on one side for too long it may start to flatten and you don’t want that. Simply place it on a different side or roll it gently in your hands to reshape it like you would a meatball.


Moss balls naturally inhabit the cool depths of lakes and receive minimal indirect light. You should mimic this environment for a healthy marimo. Keep moss balls in shaded tank areas and out of direct sunlight. Overexposure to light will lead to browning or brown spots, especially if the water gets too hot. Rotate to another side if browning is occurring from too much light.

Water Changes and Cleaning

Each time you perform a water change or fully clean your betta’s tank you should also inspect your moss ball for signs of browning or loss of color. In a container of aquarium water, squeeze the moss ball slowly allowing it to expel the water and then become resaturated.

Repeat this process several times. Think of this process like cleaning a household sponge. This will clean the moss ball and remove all of the dirt and bacteria that it has trapped over time. Brown or grey coloring is a sure sign of a dirty moss ball that is ready to be cleaned.

Sick Marimo

A healthy moss ball will be a deep and bright green color with lush outward filamentous growth. White or grey in color indicates too much light, brownish can indicate too much direct sunlight or a dirty moss ball, and black spots mean decay (remove them by cutting them out). For treatment you can also quarantine moss balls and add aquarium salt and colder water to promote health and growth.

Moss Ball Reproduction

Moss balls can reproduce in two distinct ways: naturally or forced. Growing at only 5 mm per year, marimo can take a long time to get large enough for reproduction. Natural reproduction involves a small lump starting to grow off of the existing marimo which will eventually fall off and grow on its own.

If you have a larger moss ball however, you may want to propagate it by squeezing all of the water out of it and dividing it in half or even thirds or quarters depending on its size. To promote circular growth you should continue to rotate it in the tank and carefully roll it in your hands occasionally.

Where to Buy Moss Balls

You can buy marimo moss balls in a lot of the same places as you can betta fish. Look in big pet stores like Petco and PetSmart, aquarium forums, local aquatic stores, and online. Moss balls that are not housed with other fish or plants are usually best if you don’t want to quarantine them before adding them to your betta fish’s tank. When buying them in person make sure to inspect for signs of health (e.g. bright green color).

If you’re buying a moss ball online, make sure it is from a reputable source and it’s in fact a real marimo moss ball. Fake ones do not provide much benefit aside from being visually appealing. Always check the description for size too as they can range from 0.5 inches to 5 inches or more, and will be more expensive the larger they are.


Are there fake moss balls?
– There are fake moss balls sold online and in pet stores. Fake moss balls are usually plants wrapped around a solid plastic or other material core. They are usually perfect spheres and do not have the lush appearance of a real marimo moss ball.

Why is my moss ball floating?
– Moss balls will float if they are not saturated with water or if they have an internal air bubble.

What does marimo mean?
-Translated in Japanese, Marimo means ball seaweed.

Still have a question or just want to share your experiences with marimo moss balls and betta fish? Share them in the comments below!

7 Responses

    • Bryan

      Yes, Jenny, it should be okay. If you notice it starting to brown, move it to a place that has more shade in the tank.

  1. Ginna

    Hi, I have a betta fish a year ago, since I have had fungi, I have tried several products and remedies that used to and works a lil but a few days ago my fish has white fungus, is long down the tank or on the surface, is swimming slower, does not eat well, I do not know how to help him, when they have fungi suffer? Can I do something to help him?


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