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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?

This guides dives into everything you need to know about the Marimo moss ball, including care tips, optimal water parameters, tank mates, propagation, and more!

Important Update: Authorities have recently discovered Zebra Mussels, an invasive species that poses risk to natural habitats, in Marimo Moss Balls in several states. Please make sure you check your local laws before purchasing any Marimo Moss Balls.

What is a Marimo Moss Ball?

While a Marimo moss ball looks like a small green plant, or maybe a moss, 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; it is simply algae all the way through.

Before you get worried about adding algae to your tank, this algae is not capable of spreading and growing like normal algae does.

The growth of this algae is extremely slow, and it is not 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 the tank.

While it does not naturally attach itself to decorations or other parts of the tank, it is possible to glue the Marimo to various areas of the tank as decoration, though this can cause long term damage.

While it is possible to create extremely attractive “grassy” areas of the tank by gluing multiple Marimos to one area, or creating faux trees with them, it can cause them to rot.

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

Why Do They 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 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, or multiple Anubias, or several large bunches of Brazilian water weed.

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.

While java ferns and Anubias also have slow growth rates, large production firms can increase the rates of 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. This is extremely slow growth, and since they are primarily propagated by asexual reproduction, it can take four or five years to grow them large enough to sell.

Due to the slow turnover rate, many are simply harvested from natural areas, which is also a costly expenditure.

Whether they are grown alongside other plants or harvested, it is very costly to grow them or produce new ones, resulting in higher costs.

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

In terms of temperature, these moss balls normally come from very cold water, but seem to do very well in any range normally seen in aquariums, though they can start to melt and rot if the temperature reaches over 80 degrees.

They can survive in a wide range of pH, but it seems like a more neutral or slightly acidic pH is best for them.

As far as lighting goes, they are not picky. Whether the lighting is low, medium or high, they will do well in the tank.

Water hardness also does not seem to play a large role in the overall health of a Marimo moss ball.

Their adaptability to nearly any type of water is one of the reasons that their popularity is rapidly growing.

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 naturally roll around on oceanic floors, which helps them keep their shape.

Since they are not normally rolling around in aquariums, they can begin to develop abnormal shapes.

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 decoration. 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 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 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.


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.

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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