How Many Marimo Moss Balls Per Gallon? (& Why You Might Want to Hoard Them)

One is never enough.  You might be asking yourself, though:

How many is too many?

It is safe to add 1 – 3 marimo moss balls per gallon of water. The main limiting factors are light and space to put them. For a 20-gallon aquarium, you could have 20 larger marimo or up to 60 smaller ones if you have adequate floor space.

The 1-3 gallon number is just a rule of thumb, however. Let’s figure out how many you can fit in your individual tank.

Way too many marimo.  Keep to between 1-3 marimo moss balls per gallon or less

How Many Marimo Should You Add to Your Tank?

There are 2 main areas of concern when figuring out your tank’s upper limit for moss balls:

  1. Available light. Marimo don’t need a whole lot of light.  They do need some light, however.  If you have lighting that’s coming in at an angle, you want to make sure that your Marimo aren’t shading each other.
  2. Available floor space.  Obviously, if the bottom of your aquarium is covered in moss balls to the point where you’re starting to stack them on top of each other, you might have a problem.  Also keep in mind that they float and can relocate if you have enough of a current (or curious tank mates).  You want to make sure they’re not crowding each other out.


Marimo will do best in a bright, indirect light. This means a few things.

  1. You need to keep them out of direct sun light. This can cause them to turn brown. (If they are starting to turn brown, by the way, check out my guide here on what to do.)
  2. They do need enough light to stay green and healthy.

If you have extremely bright lighting (or direct sunlight) in your aquarium, you may be limited to spots that are shaded by other plants for where you can put them.

If your aquarium is limited to the standard hood lights that came with it, you will be limited to spots that aren’t shaded by anything else.

With the proper lighting, you may notice your marimo floating to the top of the tank during the light hours and sinking at night.

Clever Placement

While “floor space” is the other limitation I mentioned, you don’t have to be limited to open areas on the floor of your aquarium.

Instead, if you use a bit of imagination, you can fit a lot more marimo into a small space.

For example, you could get a piece of driftwood that resembles a tree, attach the marimo to its branches, and then plant the tree in your aquarium.

You could have one or an entire forest.

(And if you have poor lighting, this would get your marimo balls closer to your aquarium lights.)

How Big Do Marimo Moss Balls Get?

Marimo balls have been known to get as large as 36”.

That having been said, they only grow 5mm per year.

Because of this, you don’t really have to worry about them becoming huge and taking over the aquarium. They will pretty much stay the size that you bought them.

(Until you’ve had them for a looong time.)

Moving on:

With all of that said, the limit you’re most likely going to come to is an aesthetic limit.

At a certain point, you’re going to realize that adding any more to your tank just won’t look right.

The rule of thumb of 1-3 per gallon is more of an upper limit than something that you’ll necessarily want to try to achieve. (Though there are benefits of having this many marimo – which I’ll cover in detail in the next section.)

Let’s jump into that.

The Benefits of Having More Marimo

One Marimo ball can be great for decoration, but if you want to start seeing the benefits of Marimo balls, you will want to be in the 1-3 per gallon range I mentioned above. (I’ll cover why later in the post.)

And make no mistake:

These plant-like creatures have some serious benefits in your aquarium.

Here are a few:

Reduction of Polution AND Algae

It’s no secret that fish produce waste.  (And some of them produce a ton of waste.)

The answer to this waste production is the Marimo.

Marimo absorb nitrogen compounds, like nitrate and ammonia as well as phosphates and other pollutants that can harm your fish. 

In a tiny aquarium, a few can go a long way, but to really get the benefits in a larger aquarium, you should either add a large number of them or supplement them with plants.  We’ll get more into the number of them you should put in an aquarium later on in this guide.

An added benefit of this nutrient consumption?

Less nutrients available for unwanted forms of algae to grow.

This may not eliminate algae completely, but Marimo can act as a weapon in your arsenal against a green, slimy tank.

Throw Away Your Air Stone

In addition to removing pollutants from the water, they also help oxygenate the water.

You might sometimes notice them floating.  This is because they produce lots of tiny oxygen bubbles as part of their photosynthetic process, causing them to float.

This helps aerate the water (benefiting your fish) and can help replace the need for an air pump in some circumstances.

Tough as Nails

Marimo are one of the hardiest things you can put in an aquarium.

You almost can’t kill them.  They require almost no maintenance, no special conditions, and they can thrive in fresh water and brackish water both.

But Your Need More Than 1

Earlier in the post, I mentioned you should stuff as many marimo as you can fit into your aquarium if you want these benefits.


Because marimo only grow 5mm per year.

Since they grow so slowly, they aren’t going to take up many nutrients from the water.

This is both good (low maintenance) and bad (less benefits than faster growing plants).

You can substitute them for other plants, however, if this seems a bit excessive.

Caring For a Tank Full of Marimo

Luckily, these things are extremely easy to care for, and having a bunch of them isn’t likely to add much time to your weekly aquarium maintenance.

Whether you have 1 or 100, the only regular care these plants need is:

  1. The first thing you will need to do on a regular basis is roll them – once per week.  This will make sure they stay round and don’t become flat on one side.
  2. Make sure to change out their water once every 2 weeks. This will ensure the water they are being kept in stays healthy and doesn’t run out of nutrients.
  3. You can also take this chance to rinse or swirl them in dechlorinated water to remove any debris they might have collected.

Even for a tank full of Marimo, this should only take you a few minutes to do. (I do have a comprehensive guide of everything you need to do to care for marimo on my blog here.)

In Conclusion

Marimo are great, and 1 is just never enough.

Whether you want to horde them or you just want a few, you’re doing good as long as you stay within the 1-3 marimo per gallon maximum.

There are some serious benefits of hitting that max, though, so that may be something that you want to do. (Even if it looks strange.)