Why Do Marimo Turn Brown?

  • By: SFUAA
  • Date: December 16, 2022
  • Time to read: 5 min.

Marimo are little balls of algae that are often called “moss balls” or “Japanese moss balls.” They’re very popular in aquariums and terrariums because they don’t need much care—they just sit there, growing slowly over time. But if you’ve ever had a Marimo turn brown, you might be wondering why it happened.

The most common reason Marimo turn brown that you didn’t give it enough light. Keeping your water temperature above 80F can also cause your Marimo to turn brown. Tap water can also cause your marimo to turn brown due to chlorine and chloramine content.

But there are also other reasons why this happens—and we’ll cover all of them here.

marimo moss ball edited
Marimo Moss Balls

The first reason that your marimo moss ball may be turning brown is because it’s not receiving enough light.

The first reason that your marimo moss ball may be turning brown is because it’s not receiving enough light. Marimo balls don’t require as much light as some plants do, but they don’t like to be in an aquarium that is too dark, so if you don’t have enough light the marimo ball will turn brown.

A basic aquarium light should be enough to provide adequate light for your marimo if kept on throughout the day. The next reason that your marimo ball may be turning brown is because of a lack of nutrients. Marimo balls need to be fed, but they also need to be cleaned regularly (about every two weeks) to remove any buildup of waste products. If you are not feeding your marimo and cleaning it regularly, its health can suffer.

Sunlight Can Be Harmful to Marimo

A quick note – you don’t want to give your Marimo too much light either. Marimo balls are hardy, but they will burn (and turn white) if they’re exposed to too much direct sunlight or bright aquarium lights. If you don’t have an aquarium lamp with low wattage bulbs or a window where there is no direct sunlight, then you should keep the moss ball away from any bright lights that could cause it to turn white or be overtaken by harmful algae.

The best way to give your Marimo the right amount of light is to place it in a shaded area. A small amount of indirect sunlight isn’t harmful, but too much direct sunlight can bleach the algae and cause it to turn white. If your marimo turns completely white, don’t panic! As long as you move it to a dark area and keep it in water that is clean and well filtered, it will go back green again within a few days or weeks.

Your tap water may also be the culprit if your Marimo is turning brown!

If your tap water is the culprit, there are a couple of things you can do to give your Marimo a fighting chance.

  • If you’re using tap water, treat it with a dechlorinator before letting it sit in a bucket or bowl so that any chlorine present can evaporate. Seachem Prime is an example of something you can use to remove harmful chemicals from the water you’re adding to your tank.
  • You can also use bottled spring water to keep your marimo in (if you’re just keeping a small tank or jar for your marimo without fish in it). Avoid using RO water, though, as it lacks the minerals necessary to help keep your marimo healthy.
  • If the water is heavily chlorinated or your live in an area that treats water with chloramine, consider using filtered water instead. This type of water doesn’t contain any chemicals that could be toxic to marimo moss balls and will also be more effective at removing other chemicals that could be harmful to your moss balls.

If you have a water softener, you should also consider using filtered water instead. A water softener can leave behind minerals that are bad for aquatic plants and moss balls, so it’s best to avoid them if possible. If you don’t have one or don’t know how to use yours, look up instructions online or ask someone in your area who has experience with them.

If the water temperature of your tank or bowl is too warm, this could also be a reason for the browning of your Marimo.

A common reason that the moss balls turn brown is because of your tank is too warm. The ideal temperature range for a Marimo ball to thrive in is between 68 and 80 degrees Fahrenheit (20-27 degrees Celsius). If you live in a warmer climate, try to keep your water temperature around 77F.

One way to keep your Marimo nice and cool if you live in a warm area is to add ice cubes to the tank. This can help keep the temperature of the water at a cooler level, which will prevent your Marimo from turning brown.

If your Marimo has turned brown from being in a tank that is too warm, don’t worry. It is possible to bring them back to their original color by lowering the temperature of your tank.

Danger: Using Algae Killing Products

Marimo are a type of algae that grow in the wild. They are soft, fuzzy balls that resemble small plants, which can lead to a big problem:

Because they are actually algae (not plants) marimo can turn brown if you use an algae killing product in your aquarium. Many people who have marimo in their aquariums use these products to keep their tank clean and healthy, but this can be harmful to your marimo.

Marimo is a type of algae, and products that remove algae from your aquarium will kill your marimo. If you want to keep your marimo healthy and green, you should avoid using these products in your tank.

If Your Marimo Is Turning Brown in Spots, It May Need Cleaning

If your marimo is turning brown in spots instead of all over, it may need cleaning:

Remove your marimo from your aquarium and gently swish it around in a bucket of dechlorinated tap water. This will remove any debris that may be attached to the ball as well as dislodge any algae growing on its surface.

Gently roll your marimo between your hands and return it to your aquarium where it can continue to absorb nutrients from the water column.


If your Marimo is turning brown, there are many reasons why this could be happening. The most common reasons are that your marimo moss ball is not receiving enough light or the water temperature of your tank or bowl is too warm. But don’t worry! There’s always hope for these little guys because they’re resilient creatures who can bounce back from just about anything! As long as you provide them with the right conditions, they’ll soon be back to their normal coloration again…or even better than before!

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