Marimo, a Japanese word which literally translates to “seaweed ball” are not moss at all - they’re actually a rare form of spherical algae. There’s no central stone or kernel inside the sphere - it’s solid algae, radiating from the center, growing radially outward at the snail’s pace of just 5mm per year. But in their natural habitats - few fresh water lakes in Japan, Estonia, Iceland, Scotland and Australia - they’ve been known to grow to between 8 and 12 inches in diameter!


Marimos don’t have many requirements when it comes to housing, but there are a few things to consider. First off, don’t keep your Marimo in a spot that gets a lot of sun. They naturally appear on the bottom of lakes where it’s quite dark, so they don’t react to over-exposure to direct sunlight well.
Another point to keep in mind is that, although some aquarium inhabitants like shrimp and dwarf crayfish love to forage on and hide behind a Marimo, there are also creatures that love it a bit too much and will attempt to eat or destroy it. Plecos aren’t good Marimo tankmates. Goldfish usually don’t make a good match for a Marimo either because they love to eat algae and bigger types of crayfish may also destroy it.

Water Changes:

Whether you keep your Marimo in a bowl by itself or in an aquarium, water changes are always important. During the summer, try to do a water change at least once a week – change all the water in a bowl, or around 25% in a filtered aquarium. Tap water is fine for this. This prevents dirt from accumulating on the Marimo.


A clean Marimo is a healthy Marimo! It’s a good idea to pick up your Marimo once in a while (especially if dirt particles have accumulated on it or if it has turned a bit brown or greyish) and gently wash it by squishing it a few times in a container with some clean water.

You can then roll it around in your hands a bit to prevent it from falling apart from the squishing; this should be done very carefully. It helps the Marimo maintain its nice round shape, which it would naturally get from being rolled across the lake floor by the current. This unfortunately doesn’t happen in a bowl, vase or aquarium, so it’s a good idea to help it out a bit once in a while by re-rolling it yourself.

Sick Marimo:

Although Marimo balls can withstand a range of temperatures and water condition, they may turn a strange color – this is an indication that something is wrong.
Marimo turning white / lighter means that it is probably receiving too much light. If the Marimo also seems a bit slimy or if its texture seems otherwise unusual, you may be dealing with a hostile type of algae growing around it. These algae choke the slow-growing Marimo, so it’s best to carefully wash them off or remove them with tweezers.

Marimo turning brown may be a sign that it’s time to gently clean it. If this doesn’t fix it, try carefully picking off the brown (dead) parts and adding a bit of salt to the water to stimulate the Marimo’s growth. If the bottom of the Marimo has turned brown because it didn’t receive light for longer periods of time, be sure to start rolling the Marimo around a bit more often to prevent parts of it from dying off again.

Marimo turning black and/or falling apart Marimo are unfortunately known to start decaying from the inside out sometimes, especially when it has been covered by hostile algae for a while or when it is simply too big for clean water to reach the inside. In order for it to become healthy again, the black parts should be removed and the Marimo should be gently re-rolled. It’ll be smaller than it was before, but it now has a good chance of surviving and growing back just fine

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Marimo Balls

  • Brand: Home Aquatics
  • Product Code: Marimo Balls
  • Availability: In Stock
  • $4.99