3 Quick Fixes for When Your Marimo Is Turning Brown/Grey/White

Caring for Marimo and keeping it healthy is probably easier than caring for any other plant or algae that you can put into your aquarium.

That doesn’t mean it’s without its dangers, though.

There are some problems that you can run into if you’re not careful about your monthly maintenance.

In this article, I’m going to cover the health problems your Marimo might face and what to do about them. 

You’ll learn to recognize what is wrong with your Marimo based on what color it’s turning or what its appearance is.

Quick Guide

My Marimo Is TurningWhat does it mean?How do I fix it?
Uniformly BrownMost likely, this is a sign your marimo is getting dirty and needs to be washed.Take it out and rinse it in dechlorinated water.
Brown on One SideYour Marimo isn’t getting enough light.Make sure you regularly roll your Marimo around. If that doesn’t work, use a brighter aquarium light.
WhiteYour Marimo is getting too much light.Reduce the amount of light in your aquarium.
White SpotsYour Marimo is likely being attacked by a parasite.Clean the marimo as best you can, and follow the instructions below.
Slimy GreenYour marimo is being attacked by another form of algae. Clean the marimo as best you can, and follow the instructions below.
BlackYour marimo is dying and decaying.Remove the black portions and roll your marimo back into a ball. Read the rest of the article to find out why your marimo was dying, and fix the root cause.

Let’s start with the most common:

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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.

Give it some time with this new setup, and it should green right back up.

Why is There White Stuff on My Marimo?

If your Marimo is turning white, this means you’re giving it too much light.

(Note – If it’s growing white spots and is slimy, read the next section on invading algae.  This could be a sign of that, rather than direct damage by light.  The solution to that is partly the same one, however, so read the rest of this section as well.)

If you have your tank sitting in a location where it’s receiving direct sunlight, either move your tank or find a way to give your Marimo some shade.

Too much light can kill your Marimo, so you want to fix this as quickly as possible.

If you’re just using an aquarium light that’s too bright, try dimming it or (if not dimmable) replace it with a lamp that produces less light.  If it’s in a tank with high light plants and it’s turning white, you may try to either use the plants to shade it or move it to another tank.

As long as you catch this and fix it, your Marimo should recover with no problem.

Danger: Slimy Marimo

Is your Marimo turning slimy or growing a layer of something across its surface?

This means that it is being attacked by another type of algae.

This requires intervention, or the foreign algae could eventually kill it.

There are a few things you can do to help with this:

  1. Move it into another container with clean water. 
  2. Clean the algae off of your Marimo.
    1. You can do this first by rinsing it off and dunking it – as described above in the section on brown Marimo.
    2. Manually clean any algae left on your Marimo using forceps.
  3. Leave your Marimo in clean, brackish water until you’ve completely removed the invasive algae from it and are sure that it’s healthy again.

You also want to start treating the cause of the excess algae growth.  This means improving your water quality by reducing the amount you feed your fish and cleaning your substrate to remove waste buried at the bottom of your tank.  You might also want to reduce how long you leave your aquarium lights on down to 8 hours.

While this invading algae isn’t the end of the world, it is important that you give it proper attention before it has a chance to seriously take hold.  The longer you let it sit, the more work you have to do and the higher chance there is of it doing lasting damage to your Marimo.

Marimo Turning Black?  This Might be the End

If your Marimo is turning black and/or falling apart, you have a serious problem on your hands.

This is a sign that the algae is dying (and decaying).

This is usually caused by one of the above conditions being left unchecked for too long – resulting in permanent damage.

The solution to this is – unfortunately – to remove the black sections and re-roll what you can save into a smaller ball.

Once you do that, you should figure out what might have caused the die off in the first place.

Review the above sections and see if there are any changes you might need to make.  Also consider whether you might have put anything in the water (such as chemical algae removers) that might have hurt the Marimo.

As long as you can fix the cause and there is still something left alive, you should still be able to help your Marimo recover.

Why is My Marimo Floating?

Luckily, this isn’t actually a sign of something being wrong with your Marimo.

This is actually a good thing:

As part of its photosynthetic process, it produces tiny oxygen bubbles.  Some of these bubbles get trapped inside it, causing it to float.

This floating may help it collect more light than it would be able to at the bottom of the lake/tank.

There are a few things that increase the chances this will happen:

I would expect that if the bubbles are a byproduct of photosynthesis, causing it to produce more bubbles would mean that it is also now producing more energy.  (That’s just speculation on my part, though.  Following the above advice can’t hurt, though.)

Plus it looks cool.

In Conclusion

While there aren’t a ton of issues you’re ever likely to run into when caring for Marimo, there are a few.

Luckily, they’re easy to spot. (And, since Marimo grows so slowly, you’re likely to have plenty of time to address the issue before it’s too late.)

As long as you keep your eyes open, take good care of your marimo (which I wrote a guide on that you can find here on my blog), and fix issues as they come up, your Marimo should be with you for a lifetime.