Marimo Moss Ball Size – How Big do They Get?

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Marimo moss balls are a nice, colorful addition to aquariums and ponds. Besides looking beautiful, moss balls are also low maintenance and easy to propagate. Once you have one moss ball, you can always create more.

The only downside is that Marimos grow so slowly. When propagating your plants, it will take a while to get a full-sized moss ball.

Keep reading to learn more about moss ball growth and sizes. I’ll teach you some interesting facts about this plant and useful ways to encourage faster growth.

How Big do Marimo Moss Balls Get?

When kept in an aquarium, the average Marimo moss ball will grow to a diameter of 2-5 inches. So, realistically, you can expect an aquarium moss ball to be about the size of a billiard ball.

The ones that grow really big will get to the size of a shot-put ball. The full potential size of a moss ball is not far from the sizes you can find in stores. Usually, the Marimo balls you buy will already measure 2.4-3.0 inches in diameter.

In the wild, moss balls can grow a lot larger. A Marimo in its natural habitat will eventually measure 8-12 inches in diameter. On the lower end, that’s the equivalent of a volleyball.

A 12-inch diameter moss ball would be roughly the size of a medicine exercise ball. Now that’s impressive!

How Fast do Moss Balls Grow?

Don’t expect to see your moss ball expand day by day. These plants grow very slowly, only up to 5 millimeters per year.

It will take years for a moss ball to reach tennis ball size. Moreover, the growth rate will slow down as the moss ball ages.

Environmental factors can also affect how quickly or slowly a moss ball grows. Things like tank size, water temperature, light exposure, nutrients in the water, other plants in the tank, and so on can influence the growth of moss balls kept in aquaria.

What is the World’s Biggest Moss Ball?

Moss balls live for over 200 years. That’s a lot of time to grow. So, you might wonder, “What’s the largest moss ball in the world?” Well, it’s said that the largest moss ball recorded up to date has a 37-inch diameter.

That’s a little over three times the size of the average large moss ball in the wild.

Allegedly found in Lake Akan, Japan, this gigantic moss ball is estimated to be 100 years old.

However, know that this information is currently unverified. I dug around a lot for the source of this info but only found this factoid here and here.

How to Make Your Moss Ball Grow Faster?

Certain environmental factors can stunt the growth of your moss ball. If that’s the case, you must rectify these problems to help the plant reach its full growth potential.

Here are some of the things you can do to boost the growth rate of moss balls:

– Set the right water temperature

Moss balls are sensitive to fluctuations in water temperature. They also have a narrow temperature range where they grow best.

Ideally, the water should be mildly warm, roughly 70-77 °F. Water that is too cold or too hot can stunt the moss ball’s growth or even kill the plant altogether.

– Provide sufficient light exposure

Plants need light to photosynthesize and produce energy. It’s a valuable resource they use to fuel their growth.

However, too much of a good thing can also become an issue. Moss balls will suffer from both insufficient and excessive light exposure. 

Moss balls do best with low to medium indirect light exposure. Avoid high or direct light, as the excess light can burn the ball’s surface. Too little light will cause the plant to starve and potentially die.

– Keep the moss ball clean

Moss balls tend to gather debris. They can also get algae growth covering the surface. Anything that can gunk up the plant will impede the proper exchange of CO2 and oxygen.

As the plant can’t take up CO2 efficiently, its growth slows down, and it begins to suffocate. So, remember to clean and rinse the moss ball weekly to avoid these issues.

– Clean the aquarium regularly

Besides the moss ball itself, you should also keep the aquarium squeaky clean. This means changing the water at least once every two weeks and cleaning the glass and substrate. Unsanitary conditions could otherwise stunt plant growth in many ways.

Most commonly, an unclean aquarium is a hotspot for parasitic algae and fungi. Moss balls are vulnerable to these organisms.

Once infected, the plant will stop growing. Affected portions often start rotting and falling off.

– Use a liquid fertilizer

Moss balls have very low nutrition needs. Thus, you don’t necessarily need fertilizer. If you keep your moss ball in a fish tank, the naturally-occurring waste in the aquarium is enough to feed the plant.

However, a little extra nutrition can only boost the growth rate. Moss balls can benefit from fertilizers that contain nitrogen, phosphorus, chlorine, sodium, and other trace minerals.

– Add CO2 to the tank

CO2 is the foundation of all plant nutrition and growth. It’s as important for plants as calories are for humans. You can imagine what would happen if humans couldn’t get enough calories to sustain their growth or body mass.

The same happens to plants in a low CO2 environment— stunted growth and a lack of vitality.

CO2 supplementation is all the more important if you have a heavily planted tank. You must ensure there’s enough “food” to go around for all the plants in the aquarium.

The easiest way to add extra CO2 is by using liquid CO2 straight out of the bottle. However, a CO2 injection system is even more effective.

How to Maintain Marimo Moss Balls?

Marimo moss balls are low-maintenance, which makes them popular with aquarium enthusiasts all over the world.

You don’t have to do much to keep this grassy ball healthy, and moss balls have lower light and nutrient requirements than other aquatic plants.

Here’s how to do it:

  1. Maintain suitable water parameters. The water values should be 70-77 °F, 6.0-8.0 pH, and up to 1.015 SG. You’ll need a heater, an aquarium filter, and a refractometer to achieve these values.
  2. Provide low to moderate indirect light exposure. Keep the aquarium away from direct sunlight to avoid overheating and algae bloom.
  3. Clean the moss ball once a week. To do so, transfer the ball into a container filled with lukewarm, dechlorinated water. Squeeze and roll the ball gently. Be careful not to damage the plant.
  4. Do 50% water changes twice a month. If you keep fish, you should also siphon the substrate and wipe the aquarium walls and decorations during each water change.
  5. Switch the ball to a different position regularly. Don’t let the moss ball lie on only one side for too long. It needs even light exposure from all angles. If parts of the ball are shaded for too long, they can begin to brown and rot.

I should also mention tank mates. If you keep the moss ball in a fish tank, be wary of large or herbivorous fish.

Goldfish, Oscars, Silver Dollars, Plecos, and certain Cichlids will bite and destroy the moss ball while feeding or playing. Opt for small, non-herbivorous fish instead if you want your moss ball to live a long, healthy life.


Marimo balls are long-lived, low-maintenance, and easy to propagate. The only downside is that they grow very slowly.

Typically, you can expect a growth of 5 millimeters per year! Still, these plants can reach a considerable size of up to 12 inches in diameter in the wild.

The moss balls you buy at the store are typically 2-3 inches in size. These aquarium moss balls can grow up to 5 inches in diameter, or roughly the size of a shot-put ball.

You can encourage better growth by maintaining suitable water parameters, light exposure, aquarium hygiene, and supplementing with liquid fertilizer.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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