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Marimo Moss Ball Care: The Compete Guide

Marimo moss balls have been growing in popularity in the aquarium hobby.

But what exactly are these small green blobs, and what is so appealing about them? Marimo moss balls are aquatic algae that are very different from every other aquarium plant out there.

Marimo Moss Ball

This guide dives into everything you need to know about Marimo moss balls. We will discuss care tips, optimal water parameters, tank mates, propagation, and more.

What is a Marimo Moss Ball?

While a Marimo moss ball looks like a small green plant it is actually a spherical form of algae. In fact, “marimo” is a Japanese term that roughly translates to “algae ball”.

While many circular growths have a center, the Marimo does not. Each round shape is simply algae all the way through.

Aquarists sometimes worry about adding marimo moss balls to an aquarium. After all, they are a kind of algae. So won’t they grow out of control and green every hard surface?

Fortunately, Marimo balls are not capable of spreading in your fish tank and growing like normal algae does.

Marimo moss balls grow very slowly, and they aren’t capable of spreading randomly to other parts of the tank. It can only grow in clumps and does not attach itself to other objects in a fish tank.

Marimo moss balls won’t attach themselves to decorations or other parts of the tank. But it is possible to glue the Marimo moss balls to decorations. Just know that this can cause long-term damage because they evolved to roll freely with water currents.

You can even create very attractive “grassy” areas of the tank by gluing multiple Marimo balls to one area. Faux trees with them are also popular with aquascapers. But eventually, the Marimo moss balls will rot.

Even though Marimo ball algae is extremely hardy, it is prone to rot under certain circumstances. Rot is the main killer of Marimos in aquariums, and it is mostly caused by incorrect care or water parameters.

Why Do Marimo Moss Balls Cost So Much?

When you first go to buy a Marimo moss ball, you may find yourself surprised at the price. They are often $6-8 for just one Marimo moss ball, or $10 for three mini moss balls.

In the big picture, this isn’t really that much. But for this price, you could get multiple java ferns, multiple Anubias, or several large bunches of Brazilian waterweed.

So why does this algae cost so much when compared to common aquarium plants?

The primary reason for the price is the slow growth of this algae.

Java ferns and Anubias also have slow growth rates. But large production firms can speed up growth with fertilizers, high lighting, and ideal water parameters.

However, fertilizers have little to no effect on the growth of a Marimo Moss ball.

These moss balls normally only grow one-half to one centimeter per year. It can take four or five years to grow them large enough to sell. Because they take so long to cultivate, wild harvesting is very common.

Marimo Moss Ball Care

marimo moss ball
Piotr719 [CC BY-SA 3.0]

Here are a few things you should know about caring for Marimo Moss Balls.

Water Parameters

\Water temperature is the most important element to successful Marimo moss ball care. Most aquarists don’t know this but they grow only in cold water habitats. Marimo balls are only found in mountainous regions in Japan and Northern Europe.

Here the alpine climate is ultracold and the glacial meltwater is very pure so the water is always cold. Temperatures above 75℉ are stressful for them and your Marimo moss balls will stop growing. If the temperature remains high they will eventually die.

Marimo balls are more flexible when it comes to water pH. but they prefer neutral to slightly alkaline conditions (pH 7.0+) whenever possible.

As far as lighting goes, they are not picky. Whether the lighting is low, medium or high, they will do well in a fish tank.  But only keep Marimo balls in high light environments when there are other plants to compete with algae growth for nutrients. Even high lighting won’t force Marimo moss balls to grow faster.

Marimo Moss Ball Care Tips

Since Marimo Moss balls are neither plants nor animals, their care needs are a bit unique.

While most plants can simply be planted, fertilized, and forgotten about, the Marimo need slightly more intensive care.

It is recommended to roll your Marimo at least once a week. You can press it in your hand and roll it to keep a circular shape, or simply flip it every week in the aquarium.

They normally roll around the bottom of alpine lakes as currents push them along. This continual rolling helps them keep their round shape and ensures all sides get access to light and nutrients. Since they are not normally rolling around in aquariums, they can begin to flatten out.

While the abnormal shape is not normally an issue, there is another issue that may arise – the bottom part of a Marimo will rot if it is not rolled frequently enough.

While they only need minimal exposure to light to live, the bottom part would not be exposed to light at all. If you wait too long to flip or roll your Marimo, you may find that the bottom part has begun to rot.

If the rot is not too bad and only some slightly discoloration, you can leave it on, as it may be able to recover.

On the other hand, if the rotten part has turned a deep or light gray, you should try to separate that section, roll it into a ball, and place it elsewhere in the tank.

If there are no signs of recovery after a few days, toss the rotten part to avoid an ammonia spike.

How to Clean your Moss Ball

As they roll around, or as the water current brings particulates over to them, Moss balls tend to accumulate mulm and crud, similar to a sponge filter.

While a sponge filter cannot do anything with the mulm, Marimo Moss balls can use some of it. However, they often take in quite a lot more than they need and should be cleaned every few weeks.

To clean a moss ball, simply take it out of the tank and place it into a separate container of dechlorinated water.

Squeeze it multiple times in the water, and you should see mulm coming out of it. After squeezing it, you can reshape and place it back into the tank.

Note that after squeezing it, most of the water is also squeezed out of the Marimo.

This can cause it to float for several minutes to several days, which normally is not an issue. As long as it is able to sink again after 2-3 days, no problems will arise.

Quick Revival

Every now and then a Marimo will seem sick for no reason.

Your water quality may be absolutely perfect for them, everything else in the tank is thriving, your plants are growing quickly, but the Marimo is just sadly sitting in the corner, slowly turning brown or gray.

Even with frequent rolling, reshaping, cleaning, and good lighting, it just seems to rot away.

Even though Marimo moss balls are able to survive most water temperatures, they can occasionally become sick in water temperatures over 70 degrees, though this is uncommon.

To fix this problem, take your Marimo out of the tank and place it in a jar or other container of tank water.

Next, place it in your fridge overnight. Take it out in the morning and place it in or near a window.

Leave it in a cooler area, or just room temperature, where it is able to receive direct sunlight. After just one or two days, there is normally a significant increase in the health of the Marimo.

Common Uses of Marimo Moss Balls

Marimo moss balls are commonly sold as “betta buddies”, “tank enhancers”, and “shrimp buddies”. So, what are they normally used for?

For the most part, these algae balls are just decorations. They grow incredibly slowly, so they do not change much and can be used as simple decoration.

While they will convert some nitrates into energy, it will not be enough to make any difference on the overall nitrate levels.

Some bettas seem to enjoy having them around, often resting on or near them, or even pushing them around the tank.

Shrimp absolutely love these moss balls and will sit on them for hours, picking at the mulm they collected. For shrimp, these moss balls are a feast.

Marimo moss balls can also be used in the creation of “Bonsai trees” with quite amazing results!

Tank Mates

When it comes to tank mates, most fish and invertebrates are perfectly safe to keep with the moss balls. However, some fish will develop a taste for them, and then you’re out the money you spent.

For example, while a carnivorous fish like a betta fish won’t eat them, a massive herbivorous fish like a goldfish will devour one in just a few minutes.

It is recommended to keep at least one Marimo Moss ball with freshwater shrimp due to the amount of biofilm that grows on the moss ball. Much of a shrimp’s diet is biofilm, so having a constant source is always beneficial to their growth and health.

Since moss balls cannot possibly cause harm to fish or other creatures, the only restriction on tankmates has to do with the safety of the moss ball.

Pros and Cons of Marimo Moss Balls

Here are a few common benefits and drawbacks of owning Marimo Moss Balls.


Even though Moss Balls primarily act as a form of decoration, they have the benefits of normal decorations as well as other benefits that normal decorations lack.

Just like any other decoration, fish within a certain size limit are able to hide behind or among Moss balls, which will help them feel more secure.

They can also increase the overall aesthetics of the tank, especially if the tank has a natural theme. You can even cut them up and glue them to various areas of the tank, creating “moss” walls, “grass”, and even “trees”.

However, if the bottom part of the Marimo doesn’t get any sunlight, it will rot.

They also help reduce nitrates in the aquarium, though not to an extent that would impact the frequency of water changes.

They will be taking out a very small portion of the nitrates, ammonia, and nitrite available, which can be beneficial in the long run.

As previously mentioned, they do need to be cleaned. However, this makes normal tank maintenance easier in a way.

They collect a great deal of mulm and hold it until you can clean them, which can lessen the amount of time you have to spend spot cleaning your tank.

Last but not least, they promote the development of biofilm and microorganisms. While this may not impact a tank that only has large fish, it is incredibly beneficial to shrimp, fish fry, and small fish varieties.

Some of them are able to eat the microorganisms as well as the biofilm that the Marimo grows.


The only negative that can arise from a Marimo Moss ball is caused by improper care. Since they are generally large and dense, if they were to rot and die, it could cause a severe ammonia and nitrite spike.

Ammonia and nitrite are leading killers of pet fish, and no fish is immune to them.

If you see signs of rot on yours, be sure that you are cleaning it every few weeks and rolling it once a week. Ensure that the water temperature is not too high, and if there are seemingly no causes, use the fridge method for a few days to try and revive it.

You can also remove the rotted areas and place them in a cup or bowl by a window, which may help revive that part. If not, at least it didn’t release ammonia in the main tank.

Signs that your Moss Ball is Dying

Since the death of your Moss Ball can have a significant impact on the overall health of your tank, be sure that you know how to identify early signs of death.

Keep in mind that the natural lifespan of this algae ball may very well be over 100 years, and due to their hardy nature, most can come back from significant damage.

The earliest sign is normally discoloration; not necessarily changing colors, but if some areas start to become lighter than others, there is a chance that your moss ball is dying.

If the Marimo begins to turn brown, yellow, or gray, it is a sure sign that your moss ball is dying. However, if this area is one that does not often see light, it is simply dying due to a lack of sunlight.

This algae makes its own food through photosynthesis, which requires an input of light and nutrients (waste). If it is unable to get the light it needs to make food, part of it will begin to rot and die.

However, if this part was already receiving light, there is likely something wrong with the chemistry of your water that is causing the issue.

Even though they do not directly require fertilizer, they still need to be able to access certain minerals found in normal tap water.

Of course, their tap water should be dechlorinated, as the chlorine can damage them and any other inhabitants you may have with them.

Try increasing the frequency and amount of water changes you are doing. This removes water that has more depleted minerals and replaces it with fresher water.

Normal tap water has enough minerals for this algae to survive, but it must be changed weekly to ensure the Marimo is receiving the proper amount of minerals.

How to Propagate your Moss Ball

Since some consider them expensive, wouldn’t it be great if you could propagate your own moss balls?

Propagation normally refers to breeding animals or plants, but the Marimo moss ball is capable of asexual reproduction.

While they will not reproduce on their own in an aquarium, it is possible for you to propagate them. All you have to do is divide them into one or more smaller pieces.

Roll the smaller pieces and try to keep them as compact as possible, and place them back into the same aquarium, or into separate ones.

Over time, these will all grow and develop into separate little spheres. While they will often grow lopsided at first, reshaping them multiple times will help them grow into perfect spheres.

Since they grow so slowly, the growth will be more noticeable in smaller Marimo Moss balls than larger ones.

Marimo Moss Ball Size

Now, you may be wondering how large these are capable of growing.

For most plants, the maximum size will determine what aquarium you need to place them in from the beginning, as it often only takes a few months for them to reach their maximum size.

However, moss balls can be moved gradually to larger tanks, or separated into smaller ones if they grow too large.

They are capable of growing up to a foot in diameter, which is quite impressive for this rare algae.

While this size is not common in aquariums, simply due to the amount of time it takes for them to grow this large, they naturally occur at this size.

In aquariums, five or six inches is normally considered a maximum size, though larger is not uncommon.

They are normally sold between ½ and 2 inches in size, though there is no particular size limit on when they should be sold. If you buy one of the larger ones, feel free to separate it into smaller ones if desired.

All in all, their care is very simple; moss ball + water. If they begin to look frail or sickly, place them in the fridge overnight and in a lit window during the day.

You can also start to remove any rotting areas to prevent the rot from spreading, and the separated areas may even make new moss balls of their own.

In Conclusion

Marimo moss balls are the most unusual aquatic plants you will ever find in the aquarium hobby. Slow growing and a little sensitive, they can live for a very long time when well cared for. They add interest to aquariums with other cold-water aquarium pets like shrimp and fish. And are practically self-maintaining, so long as you flip them over occasionally or provide a gentle current.

More Frequently Asked Questions about Marimo Moss Balls

Do you have a few more questions about Marimo moss ball care? Then let me take on some frequently asked questions about these algae balls.

Do Marimo Moss Balls Need Food?

Marimo moss balls don’t grow very fast but they do need a little fertilizer to maintain proper health. A steady trickle of ammonia from your aquarium fish is all that these slow-growing algae balls require.

How Long Do Marimo Moss Balls Live?

When kept in cold water conditions with low nutrient levels, you can expect your Marimo moss ball to out live you. They can grow for over 200 years, making them valuable in Japan as symbols for good luck and longevity.

Is a Marimo Moss Ball a Pet?

If you consider a plant to be a pet then absolutely. But since they are plants and you don’t usually “pet” slimy balls of algae, it is better to consider Marimo moss balls to be decorative additions to an aquarium ecosystem.

Can Marimo Moss Balls Live in Tap Water?

Tap water won’t harm a Marimo moss ball for short term stays. But it has no nutrients at all for growth so your moss ball would eventually starve to death. They don’t need much nutrients but a little is still required for growth and maintenance.

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

5 thoughts on “Marimo Moss Ball Care: The Compete Guide”

  1. I have a question. About 3 weeks ago I moved my aquarium. It was getting indirect light now it is getting direct sunlight for maybe an hour a day. 3 days ago right after a water change I noticed my Moss ball floating during the day and sinking at night. It has done this consistently for the last 3 days . Now I see slight brown spots evenly spaced around the Moss ball. Is this because it’s getting too much light?

  2. I have a marimo balls at home..and yes i give them a i call it once a week in the fridge…i noticed that one of then have this bump actually 2 i gently remove the bump and roll them nicely and now i have these 2big and 2small ones..did i do okay when i remove the bumps? Not harming them ?

  3. Please go to Department of Fisheries and Oceans Canada. Moss balls are carrying evasive species in them like zebra mussels and are banned in Canada.

    • Hi Donald, thank you for your comment. I updated the article to make readers aware of the current situation with Zebra Mussels being found in Moss balls.


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