Marimo Moss Ball Care Guide (+ 3 Things to Do Weekly)

Marimo moss ball care doesn’t need to be difficult:

In this guide, I’m going to teach you everything you need to know to grow beautiful, healthy Marimo Moss Balls.

This will help you avoid turning them into sickly brown things – like I (unfortunately) did with the first one I purchased.

In this article, I’m going to teach you:

  • How to keep your Marimo balls healthy and looking great.
  • The exact conditions you need to keep in your tank (ph, light, etc) to keep it thriving
  • 3 things you can do to make it grow faster

And more.

Let’s jump into it:

How to Grow Beautiful, Healthy Marimo Moss Balls

I’m going to divide this up into 2 sections:

Initial care & ongoing maintenance.

The initial care is going to show you what you should do when you first get your Marimo ball to give it the best chance of success.

The ongoing maintenance is going to show you the things you need to do every so often to keep it thriving.

ID 48047996 © Watcharapong Thawornwichian |

7 Steps to Make Sure Your New Marimo Is Healthy and Happy

When you get a new Marimo, you don’t want to immediately drop it into your existing aquarium.

This could cause some serious problems for the aquarium and any other Marimo balls in it if your new Marimo ball has any parasites.  (Other sites may tell you that Marimo doesn’t have any parasites, but this is not true.  There are a few nasties that you have to watch out for.)

Instead, prepare a cup or small, .5 gallon aquarium tank.  (This is the one thing those small tanks are actually good for.)

  1. Fill the tank with distilled water.
  2. Add 1 level tsp of aquarium salt per quart of water.  This will raise the salinity of the water, which can help your Marimo green up a bit.  (Or use this calculator to find how much salt you need to raise the water to 1.003.)
  3. Rinse your Marimo in dechlorinated water (or bottled aquarium water) to clean any debris off.
  4. If there are any slimy spots (especially if they’re white), you should clean these off by rinsing with dechlorinated water and then using forceps to manually remove any slime that’s left.
  5. Place the Marimo in your quarantine tank for 30 days – closely monitoring its health. 
  6. During this time, you might notice small worms hatching.  If this happens, try to remove any that you can, and apply Flatworm eXit or a similar product to kill any that are left.  You’ll also want to change out the water afterwards to remove any dead parasites.
  7. If this happens, you should leave your Marimo in quarantine for a few weeks extra just to make sure the problem has been taken care of.

So, while it may be tempting to go ahead and put your new Marimo into your aquarium, you definitely want to wait and follow these steps instead.

After all, it may be a (relatively) small chance that you’ll find any nasties – if you buy from a reputable source – you don’t want to find out the hard way that you got an infested moss ball.

3 Steps to Keeping Your Marimo Thriving

Once you get past the initial quarantine period, there isn’t much that needs to be done.

  1. The one thing you will need to do on a regular basis is roll them – once per week.  This will make sure they keep their shape and don’t flatten out on one side.
  2. If you’ve decided to put them into your aquarium, continue following your normal water change process.  Otherwise, you may want to change out their water once every 2 weeks.
  3. You should also take this chance to rinse them in dechlorinated water to remove any debris they might have collected.

Do this, and your Marimo balls will stay happy and healthy.

I’ll cover how to recognize health problems later on in this guide.  If any of those things crop up, you’ll need to do additional work.  Other than that, you’re set.

Next, let’s cover the conditions your Marimo needs to live:

ID 136500092 © Maurizio Biso |

The Right Parameters for Your Fish Tank

Marimo moss balls are one of the easiest things to keep alive and thriving inside your aquarium.

This means you don’t have to be too concerned about what type of conditions you’re giving it.

If you want it to be really healthy, however, you need to make sure it has the best conditions that you can give it.  This will also give you the best chance of seeing faster growth.

And it’s average growth rate is pretty disappointing, so this may be important to you.

Here are the conditions your Marimo Moss Ball can grow in:

LightAnything except direct sunlightAquarium lighting for low to mid light plants
SalinityFreshwater up to 1.015Reported to be 1.015

As you can see, almost any conditions safe for freshwater fish are suitable for Marimo.

There are some things you can do to keep them healthier and improve their growth rate.  We’ll discuss those below.

One thing I want to discuss here is salt.


Most freshwater plants don’t particularly enjoy salt. 

Marimo is a notable exception.

If your Marimo starts turning brown, one of the things you can do to help green it back up is to put it in a water with 1.015 specific gravity.  (This is a way to measure how much salt you have in your water, and you can use a cheap hydrometer from your local pet store to measure this.)

If you put a normal freshwater plant in this, it will likely kill it.  So unless you’re keeping a brackish water aquarium, I don’t recommend quite this much salt unless your Marimo is by itself.

A little salt will actually benefit both your Marimo and your freshwater aquarium, however.

It does this by killing the ich parasite that plagues a lot of freshwater fish.

To do this, you want to add 1-2 tsp per gallon, or (the better option) raise your water to 1.003 sg as measured with your hydrometer.

This will benefit both your fish and your Marimo – while remaining safe for a lot of freshwater plants (such as Java Moss, for example).

What to Do When Your Moss Ball is Turning Brown or Grey

If your Marimo is uniformly turning brown or grey, this probably means that it’s getting dirty.

The fix for this is pretty simple:

  1. Take it out and rinse it.  Take your Marimo out of its tank and give it a gentle rinsing using either dechlorinated and/or purified tap water or bottled aquarium water.  (You can do this, if desired, using a water bottle with some holes poked in the cap to mimic a shower head.)
  2. Dunk it. If it’s still dirty after this, dunk it into the water, lift it out, and give it a gentle squeeze.  You may also gently swish it around, being careful not to do so vigorously enough to damage it.
  3. Soak it. If after steps 1 and 2 it’s still brown, fill a cup with brackish water (1.015 sg on your hydrometer – or a literal pinch of salt in the cup) and let it soak.  This should help it green up.

If your Marimo is still brown after this, or if it is turning brown on only one side, however, this means that it’s not receiving enough light (on that side).

If you have something shading the Marimo, you may want to relocate it so that it gets the proper amount of light on all sides.  Otherwise, you may want to look at increasing the amount of light in your aquarium via an upgraded aquarium light.

How Quickly Do Marimo Grow?

You may have noticed above that I mentioned Marimo’s dismal growth rate.

Marimo grows at only 5 mm per year on average (or around 1 inch every 5 years).

To put this into perspective, if you buy a 1″ moss ball today, 36 years from now it will get to be 8″.

The oldest Marimo we are aware of – one that’s at least 200 years old – is only 36″ in diameter.

That’s not great, but if you’re looking for something that won’t require any trimming (or much maintenance at all) it makes this the perfect thing to put in your aquarium.

If you want a bigger Marimo, however, spend the extra money to buy one that is the size you want.

[Advanced] Increasing Their Growth Speed

Is there anything you can do to increase the growth rate of your Marimo?

Surely they don’t all grow at 5 mm per year regardless of what you do to them, right?

The answer to that is yes – there are ways you can increase the growth rate a bit.  It’s not a satisfying yes, however, because there is no data available on exactly how much you can speed that growth up.

You might double it, or you might go from 5mm per year to 5.5mm per year.

If you’re up for an adventure, however, I’m going to cover 3 ways to increase the growth rate of your Marimo.  Then you can find out for yourself.

Carbonated Moss Balls

The first thing you can do to help your Marimo grow faster is to install a CO2 diffuser in your aquarium.

CO2 is something that is commonly used to help both terrestrial and aquatic plants grow faster, and Marimo is no different.

Chances are, if you see someone with a really nice aquarium full of lush growth, they’re using a CO2 setup to help grow their plants faster.

Luckily, adding CO2 to your aquarium is neither as complex nor as expensive as you might, at first, think.

In terms of cost, you can have it as cheap as $25 for the CO2 setup itself and another $10-15 for the diffuser.  (This is for DIY kits that use citric acid and baking soda or yeast and grape juice, so ongoing costs are pretty low.)

For more reliable setups, you should expect to pay around $50 for the CO2 regulator + an extra $15 for the CO2 tank itself on an entry level setup.

For these, setup is about as simple as adding an air pump and air stone to your aquarium.

Setting up the CO2 kit is beyond the scope of this article, but know that it’s an option if you are looking to help your Marimo’s growth along.

Liquid Fertilizers

Another thing you can do is to add some liquid fertilizers to your aquarium water.

This will give it everything it needs to grow, and you will see faster growth rates in turn.

It’s important to note that you should avoid going down to your local store and buying any water soluble fertilizer they carry.

Doing so could seriously harm or kill any fish you have in the aquarium.

Instead, look for fish safe fertilizers at your local fish store or retail pet store chain.

Something like Seachem Flourish is exactly what you’re looking for.  This is low nitrogen and won’t harm your fish if used in the proper amounts.

Better Lighting

Your Marimo will grow in pretty much any aquarium lighting that you give it – and it will be perfectly healthy.

If you give it better lighting, however, it will grow faster.

Check out lighting designed specifically for low to mid light plants or – alternatively – try experimenting with lighting for high light plants.  (I’d recommend finding something dimmable if you go the high light route.)

If your Marimo starts turning white, you’re adding too much light.

If not, you’re doing fine.

The downside of this is that it can encourage the growth of algae in your aquarium.

The upside of this is that it can encourage the growth of algae in your aquarium.  (And Marimo is a ball of algae.)

Using a Combination

If you do one of these, it might help, but to truly get results, you’ll want to do all three.

If you use one without the others, you won’t see nearly as much results as if you use all of them.

The extra light will help it produce more energy in photosynthesis, but doing so will cause it to use more nutrients, which it will get from the fertilizer and CO2 you’re adding to your aquarium.

Can You Grow Marimo in Saltwater Aquariums?

It’s not recommended that you put your Marimo in saltwater.

Water up to 1.015 sg is fine for Marimo, but actual saltwater – which could be 1.023-1.028 – will probably kill it.

If you have a saltwater aquarium and you want to start keeping Marimo, your best bet is to start a nano tank just for your Marimo.

You can go with a 5 gallon, and if you decide to put a Betta or a few shrimp in later, you’ll be able to drop them in without upgrading your tank.

Compatible Fish, Invertibrates, & Plants

You’ve spent your hard earned money buying a Marimo (or a few).

You’ve spent your time tending to it in quarantine to make sure it’s not going to infest anything else.

Now you just need to make sure the fish (or invertebrates) you’re putting it in with aren’t going to kill it.

For this section, I’ve gone through accounts of other people that have Marimo in tanks with various creatures.

I’ve paid attention to which ones have killed them and which ones have been good tank mates.

Now, I’m passing that information on to you.

Which Fish are Compatible with Marimo Moss Balls?

When it comes to choosing fish for the tank you keep your Marimo in, there is one main factor:

Are they going to eat your Marimo?

Betta are famously Marimo friendly.  This is probably because they are carnivorous.

Goldfish, on the other hand, will destroy your Marimo.  They are omnivorous, and they are notorious grazers.

For this reason, Mollies are probably also best avoided.

You also want to avoid algae eating fish, because some of them may make a snack out of your Marimo.

There are reports, however, of plecos living with Marimo and completely ignoring them.  Your mileage may vary here.

If you avoid fish with a reputation for heavy grazing, you should be fine in this regard.

What About Shrimp and Snails?

One would think that snails and other creatures with a reputation for eating algae would also eat your Marimo.

This appears not to be the case, however, and there are a lot of reports of algae eating snails either ignoring or playing with the Marimo (but not destroying it).

Based on this, most snails and small shrimp should be good. 

It is wise to avoid larger crustaceans such as crabs and bigger crayfish.  There are numerous reports I’ve been able to find of these destroying Marimo.

Whether this is intentional behavior or just a byproduct of them hunting for food I’m not sure, but I’d keep them separated.

What Plants Should You Pair With Marimo?

Pretty much any low or mid light plant pairs well with Marimo.

It’s not very picky in this regard.

Here is a table of some good choices:

Plants to Pair with Marimo
Java Moss
Parrots Feather
Anubias Barteri
Rotala Rotundifolia
Crypt Wendtii
African Water Fern
Guppy Grass

Take Care of Your Marimo

Marimo care isn’t that hard.

All you have to do is clean it every once in a while and make sure it keeps its ball shape.

Buying it is much harder, depending on where you go, but if you know how to spot a real one from a fake one, it’s not really that difficult either.