How Many Marimo Balls Per Gallon?

Beginners and experienced aquarium owners alike will avidly talk about moss balls in their tanks. These low maintenance additions are popular for a number of reasons and do well with all kinds of fish and plants.

However, if you are planning on adding marimo moss balls to your tank, you are probably wondering how many marimo balls per gallon is safe? For starters, you want enough moss balls to balance out the aquascape. Secondly, you want enough room for all your fish and moss balls to flourish and be happy.

What you will find in this article

Today, we’re going to introduce everything you need to know about moss balls, including the rule of thumb for how many marimo balls per gallon to add to your aquarium. Let’s get started.

What Are Marimo Moss Balls?

Marimo moss balls are native to the icy waters around Iceland, Scandinavia, Scotland, and even in parts of Japan (where they are a popular “pet”). Although they are not sentient, marimo moss balls are living clumps of algae that have been packed together. They also do not have a core, so if they start to fall apart, you can technically put them back together again.

And don’t worry. While most algae is not friendly or healthy for a fish tank, these little moss balls can do a world of good for the aquascape of the aquarium.

You can purchase moss balls online or at a fish store. There are also artificial marimo moss balls available, but the real ones are highly recommended.

Benefits of Marimo Moss Balls

Marimo moss balls do more for your fish tank then roll around and look cool. These little clumps of algae have a number of positive effects for the tank and your fish.

Here are some of the benefits of marimo moss balls:

Minimal Maintenance Required

One of the reasons many aquarium owners avoid adding living plants to the tank is the care requirements.

You always have to worry about the temperature and lighting and putting enough nutrients in the water. Then, once the plants grow, you need to trim them back.

Marimo balls

That is not the case with marimo moss balls. In fact, once you drop a couple of moss balls into the fish tank, you can leave them alone and not worry about them. Once in a while, you will have to clean them or turn them over, but otherwise, you don’t need to feed them or give the moss balls any special care.

Home For Beneficial Bacteria

All tanks harbor bacteria—some beneficial, some not. Moss balls carry with them beneficial bacteria that can break down nitrites and nitrates in the water, as well as ammonia, making the environment much more hospitable for the fish. Also, the surface area of a marimo moss ball is rather large, providing the perfect place for helpful bacteria to propagate.

Thanks to the beneficial bacteria, you also will find that the tank and the filter stay cleaner for longer. That is a huge reason to add marimo moss balls to your aquarium.

Waste Removal and Oxygen Production

Since marimo moss balls are plants, they can also remove waste from the water and release oxygen. After all, water-bound plants can use the nitrates, nitrites, and ammonia in the water for photosynthesis, thereby removing toxins from the water to create energy for themselves.

This also means that marimo moss balls will remove CO2 expelled from the fish and replace it with oxygen. All of your fish will appreciate having more water circulating in the tank.

Extremely Hardy

Killing a marimo moss ball will be a challenge. These bundles of algae are very resilient, and they can survive in a range of ph levels, temperatures, and in either salt or freshwater. Even if you remove marimo moss balls from the water for a couple of days, as long as the moss stays moistened, they will survive. The only way to kill a moss ball is with chlorine or algaecides (algae killer).

Does Not Produce Own Waste

If you have other plants in the fish tank, you might notice that they can produce waste, such as shedding leaves and slime on the roots and other things. A lot of decaying matter from growing or dying plants is going to release ammonia into the water, and that can cause problems for your fish.

The good news is that marimo moss balls do not produce waste. Even though they are a plant, they rarely emit ammonia or leave a mess behind unless they are unwell or dying.

How Do You Care For Marimo Moss Balls

Yes, we did say that marimo moss balls don’t need a lot of upkeep—and it’s true. You hardly have to do anything, but you do need to cover a few basic steps to avoid killing it off.

First, you want the moss ball to maintain its round shape. That means it cannot stay in a single spot for too long. If you leave it stationary, it will sag and breakdown. To stop decay, simply roll the moss ball around the substrate once in a while. Of course, if you have a betta fish or a bottom feeder, it might knock the moss ball around for you.

That means that the only thing you need to concern yourself with is sunlight. Too much sunlight will cause a moss ball to grow brown, and too little light will also make it go brown. If you notice that your marimo moss balls aren’t faring well in the sun, move them to more shaded portions of the fish tank and vice versa.

How Many Marimo Moss Balls Per Gallon?

Keeping everything you now know about moss balls in mind, you can now start to figure out how many marimo moss balls can fit inside your fish tank. Generally, you can add 1 to 3 moss balls per gallon. That is about ¼ of your aquarium filled, if you consider that as a per gallon ratio.

No. of gallonsNo. of moss ballsIdeal size
5 gallon tank4-82.5 inches
10 gallon tank8-132.5 inches
20 gallon tank13-202.5 inches
30 gallon tank21-302.5 inches
55 gallon tank50-612.5 inches
100 gallon tank90-1102.5 inches

Moss Balls in a 5 Gallon Tank

5-8 Moss Balls

Most 5 gallon tanks are rectangular in shape and will measure around 16” L x 8”W x 10” H (40 cm L x 20 cm W x 25.4 cm H), giving you a base of about 128 sq. in (825.8 sq. cm). Considering you can have 1-3 marimo moss balls per gallon, that means you can have 5-8 moss balls in a 5 gallon aquarium. In other words, you need around 5 average sized moss balls to get to ¼ of the aquarium filled.

Moss Balls in a 10 Gallon Tank

8-13 Moss Balls

Going off the same idea as with the 5 gallon tank, a 10 gallon tank can accommodate 8 to 13 marimo moss balls. A rectangular 10 gallon fish tank usually has a base area of around 200 sq. in (1290 sq. cm). Again, to cover a quarter of the base, you need at least 8 moss balls.

Moss Balls in a 20 Gallon Tank

13-20 Moss Balls

A rectangular 20 gallon tank usually has around 288 sq. in (1858 sq. cm) as its base area. To cover around ¼ of the substrate, you need at least 13 moss balls, but you can fit up to 21 marimo moss balls if they are smaller than average.

Can You Have Too Many Moss Balls?

Source: Sendai Blog

Even though the ratio given is considered the perfect scenario, you might be wondering just how many marimo moss balls are considered overwhelming for an aquarium. Or, what if you filled a tank solely with moss balls? Well, it turns out there is a limit to how many moss balls you can fit in an aquarium. The reason being is light. When you put too many moss balls into a small space, they start shading each other. Without enough light to feed the moss, the balls underneath will brown and eventually die.

Another reason not to add too many moss balls is nutrient balance. If you overload the tank with marimo moss balls, they will not be able to draw enough nutrients from the water and will not properly grow. While this is rarely a problem if you have fish in the tank, understocked or even empty aquariums can be problematic for your moss balls.

Of course, you also want to think about the fish inhabiting the space with the moss balls. If you add too many, you cut down on the places your fish have to swim. That can be just as bad as keeping your fish in a tiny tank.

Also, you should consider the aesthetic. Too many moss balls will take away from the beauty of the other plants, disrupt the balance, and will appear too cluttered to the eye to be appealing.


In short, marimo moss balls are a wonderful addition to any fish tank and will be a delight for both you and your fish. They are extremely easy to care for, as well. If you plan on introducing marimo moss balls to your aquarium, remember that there can be too much of a good thing. So, follow the rule of thumb: Add 1-3 moss balls per gallon. In other words, a 5 gallon tank can take 5-8 moss balls, while a 20 gallon take may take anywhere between 13-21 marimo moss balls.

Have you added moss balls to your aquarium? How many marimo moss balls per gallon have you put in the tank? Let us know in the comments or send us a message.

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