The Definitive Marimo Moss Ball Care Guide

Marimo Moss Care

Marimo Moss Balls are adorable little green growths. Although they are called Moss Balls, they are actually balls of algae that grow slowly over their long lives in the bottom of lakes and rivers in the northern hemisphere.

They are easy to keep and make great conversation starters which make them a popular choice for fish keepers. If you are looking for a unique and easy to care for addition to your tank then the Marimo Moss Ball is a great choice. Keeping reading to learn more about this unusual organism…


Marimo Moss Balls are originally from Japan, however they are now also found in the cold waters of Iceland and Estonia. Their botanical name is Aegagropila linnaei and their common name Marimo came from the Japanese words Mari (which means a ball) and Mo (which means algae). Interestingly they are not moss at all and are actually made from algae.

In the wild they are most commonly found as long filaments, but in aquariums they are commonly kept in the ball shape. They are famous in Japan because of a festival held in their honor near Lake Akan every October. The Marimo traditional festival was started in 1950 to raise awareness about their need for protection.

Fortunately this has given them the fame needed to keep them safe. They were initially sold across Japan before they entered the fish keeping hobby in America and then the trend spread to the whole aquatics hobby.


Marimo Moss

Marimo Moss Balls are soft and almost fuzzy in texture. They feel very much like wet velvet and can be squished in your hand to drain them of water.

When smaller Marimo can be bright green and as they get larger they tend to go a darker shade of green. Because they are algae and not plants, they do not have leaves or roots and instead roll gently with the currents.

There are in the wild:

  • On the shaded side of rocks
  • Free-floating filaments
  • Ball shape

To grow naturally in a ball shape takes very specific requirements, but free-floating filaments can be rolled into balls by hand and these are often sold instead of wild forming Marimo. Inside a home aquarium these moss balls can be as small as 0.25 inches or as big as 6 inches. Most will grow at an average rate of 0.2 inch per year, which is slow but fast enough to recover from any small bites taken out of it by your curious fish.

In the wild some Marimo have grown to over 3 feet, but this has taken 200 years for them to get to this size. Many describe them as smart because of a peculiar behavior that can be seen in the home aquarium. When exposed to light continuously, they release oxygen which gets trapped in their filaments and rise up to the surface. This means they take full advantage of the daylight and at night Marimo will settle again.

Buyer’s Guide

When buying Marimo Moss Ball you should look for a well-rounded ball that does not have patches of discoloration or long loose strands. A round and bright green Marimo is more likely to be healthy than one which looks dirty or faded.

The best way to tell a good quality Marimo is by handling them. This allows you to test whether they are real, or just another type of algae wrapped around a hard center (which real moss balls do not have). They should be easy to mold and feel as if they are easy to tear apart – though most sellers won’t let you test that!

By handling them you can also tell whether they have imperfections such as parts that are harder or softer, or parts where the algae have clumped or broken apart (these are all the signs of a healthy Marimo). You should pay between $1-$5 for a smaller Moss, or $20+ for larger Moss. Just remember that the quality may not increase with a higher price tag, so always check the health of the Moss Balls.

How To Place Marimo Moss Balls

Marimo Moss Ball In Tank

Because these balls do not have roots they cannot be planted or submerged into substrate. They should be left to move around the aquarium with their own floating cycles and with the movement from the filter. A soft substrate is best suited for the Marimo as it allows them to gently roll around and keep a more uniform shape.

If you want them to be mostly stationary then putting them against an ornament or the glass opposite the filter output is the most likely way to be successful. When you first place Marimo into your aquarium they will float for some time and will often be investigated by their tank mates. After some time they will gently sink to the bottom and you can move them to the desired position in the tank.

Marimo Moss Ball Care

Tank Conditions

This moss naturally grows on the banks of soft sediment lakes and rivers in the northern hemisphere.

These conditions are easy to replicate in your tank. You will need cold water, a soft sediment and indirect sunlight or medium tank lights. Some aquarists like to keep them in tropical tanks, which is fine. But just be warned that Marimo might not grow and thrive. For this moss to thrive you should keep the temperature of the water below 78°F. The pH can range from slightly acidic to slightly alkaline, anywhere between 6 and 8 should be fine.

The filter you choose for your tank will determine whether the balls stay still or move around. If you want the balls to stay in one particular place then pick a filter which does not have a powerful output. However some people prefer the aesthetic of their marimo moss balls moving around, so in this case you should use a more powerful filter.


This moss is very easy to care for. In fact it might be the most easy aquarium plant to care for. Marimo Moss Balls do not need much trimming unless they begin to brown (brown parts can be removed by hand).

Apart from this all you need to do is wash them each time you change the water in your tank. You should rinse Marimo in freshwater and gently squeeze them to remove the old water before allowing them to reabsorb clean water. If the ball is getting too large they can be broken down and rolled into new balls by hand. After the water is changed it is also useful to rotate the Marimo onto a different side. By having a different side on the bottom when they are rotated, they will keep their round shape and not grow with flattened parts.

They do not need any supplements but if you are looking to improve their color or growth rate then aquarium plant food is available.

Common Problems

The most common problem with Marimo Moss Balls is color change. They can change color to brown, black, or white.

White: If the Marimo becomes white then it is being bleached through over exposure to light. You should move it to an area with less intense light and no direct sunlight so it can repair.

Black: Any parts of the Moss that have turned black are dead and should be removed from the ball.

Brown: If the brown spot is on the bottom then turning it so the brown part faces the light should fix it. Brown spots elsewhere on the Moss should be able to be washed out. If neither of these work then remove the brown spot and reshape the moss in the palm of the hand.

When removing parts of the Marimo you can put it in water that is up to 5% salt. This will encourage it to grow faster until it has grown large enough to be returned to the aquarium. You can do this in a small container that does not have direct sunlight on it.

Tank Mates and Compatibility

Most plants that are suitable for home aquariums can look amazing alongside the Marimo. Plants that are as easy to look after such as Anubias also do well in lower light levels. Another plant that pairs well is Water Wisteria. You can float it on the surface and it will block some of the light coming into the aquarium, which is ideal with Marimo Moss Balls.

Most fish that do not need to be kept in warm waters are suitable tank mates. Guppies, Platies, Tetras, and Mollies are all great tank mates as are Danios, Minnows, and Rosy Barbs. The most popular tank mate for a Moss Ball is the Betta fish, as their soft surface lets Bettas rest easily and their ability to be moved provides much entertainment to the curious fish. It has been known for Bettas to push the balls around, especially when floating at the surface.

Marimo are also very popular with shrimp keepers as the filaments trap lots of particulates that the shrimp can clean off them. As they are one of the more curious looking additions to a tank, this pairing can lead to a tank that is interesting while also being easy to keep. You should avoid any fish that enjoy eating algae. Goldfish for example have been known to tear them apart very quickly, so should not be kept together.

How to Propagate Moss Balls

Moss Balls reproduce through asexual reproduction.In the wild this is a natural process and they will either continue to grow or divide and break when they become too big.

To replicate this in your aquarium you will need to remove the Marimo from the tank and squeeze any excess water from the moss. Next you need to use a sterile pair of scissors to cut the ball in half. Depending on the size of your original Marimo ball you might be able to cut the halves again so you have four pieces.

Just make sure that the new pieces are not less than 1 inch in size. You can then use thread to help create a ball shape. Tie two pieces of string around the moss, just like you would tie a gift box up. You can then place the new balls back into the aquarium and turn the ball every few days to encourage even growth. Remove the string after a few weeks once the moss has started sticking together.

Frequently Asked Questions

Can you keep Marimo Moss Balls with Bettas?
Marimo Moss Balls make great additions to Betta tanks.

Betta fish have actually been seen to push these balls around in a play-like way, and rest on them when they are tired.

How to make a Marimo Moss Ball terrarium?
A Marimo Moss Ball terrarium is usually created using a small jar, vase or bowl.

You will need a couple of balls, a jar, vase or glass bowl and some smooth glass pebbles for the bottom. You could also add a small piece of driftwood or stones.

In Conclusion

We hoped you enjoyed learning about Marimo Moss Balls. These fascinating growths can be found in almost every aquatics store. Not only are they loved by fish keepers but house plant communities also treasure them for their unique form and often keep them in jars or old fishbowls.

Whilst they do not serve any additional purpose than plants, they do provide an area for fish to swim in and out for shelter. They are also great areas for beneficial bacteria to build up on and they catch food well making them popular areas for fish to feed from. Marimo Moss Balls add beautiful color to any tank.

Are you tempted to get a couple to brighten up your aquarium? Let us know in the comments section below…