Marimo moss balls are the easiest living thing you can keep in an aquarium — soft green spheres of algae that need almost no care, tolerate cold or tropical water, and live for decades. They are not actually moss but a rare form of spherical algae, and they make a charming, low-maintenance addition to shrimp tanks, betta tanks and nano aquascapes.
Here is how to keep them thriving.
- Marimo moss balls are spherical algae, not moss — the easiest living aquarium decoration.
- They need low to moderate light and tolerate 15–24 °C, including cold-water tanks.
- Roll and gently squeeze them weekly to keep their round shape and green colour.
- Inspect new balls for invasive zebra mussels before adding them, and buy from reputable suppliers.
Basic care
- Light: low to moderate — bright light causes them to brown or grow algae on the surface.
- Temperature: 15–24 °C is ideal; they tolerate cold-water and tropical tanks.
- Water: any clean aquarium water; they help absorb nitrate.
- Placement: rest on the substrate; roll them occasionally so all sides get light.
Cleaning and rolling
Roll them weekly. Gently squeeze and reshape the ball during water changes to keep it round and let every side reach the light. This is what keeps its dense green shape.
Rinse in old tank water if debris collects. If a ball develops a brown patch, gently squeeze out the trapped water and give it a little more light.
The invasive-mussel warning
In recent years, some commercially sold marimo balls were found to carry invasive zebra mussels. Before adding a new marimo ball, quarantine and inspect it, and if in doubt, buy from a reputable UK aquatic supplier. Never release aquarium plants or animals into the wild.
Buy healthy marimo moss balls from a trusted source.
Tank mates
Marimo balls are safe with everything and are a favourite in cherry shrimp and betta tanks — shrimp graze the biofilm on their surface. See our best aquarium plants guide for more low-maintenance options.
Frequently asked questions
How do you care for a marimo moss ball?
Keep it in low to moderate light at 15–24 °C in clean aquarium water, and roll it gently during weekly water changes to maintain its round shape and even colour. They need almost no other care.
Why is my marimo moss ball turning brown?
Usually trapped debris or too little light on that side. Gently squeeze out the old water, rinse it in tank water, roll it regularly, and give it slightly more light.
Do marimo moss balls clean the water?
Yes, to a small degree — they absorb nitrate and other nutrients as they grow, which mildly helps water quality. They also provide biofilm that shrimp graze on.
— Jaqline Smalling, FishForever
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