Spotlight: Marimo ‘Moss’ Balls

Marimo is a type of green algae native to the most of the northern hemisphere that, under very specific conditions, forms into a sphere. Colonies of these strange balls have been found washed up in Iceland, Scotland, Japan, Estonia and now Australia. They are particularly prevalent in Lake Akan in Japan where the constant wind-driven waves spin the algae into balls, with some growing to 20-30cm before collapsing on their own weight.


The name ‘Marimo‘ (roughly translating as ‘bouncy play ball’) was coined by Japanese Botanist Tatsuhiko Kawakami after their discovery by Austrian scientist Anton Eleutherius.

Here are some facts about Marimo in the aquarium.

  • In the aquarium they are very hardy plants, although do tend to photosynthesise better in shallower, colder water with lots of flow that shakes off any detritus stuck to the algae.
  • They are good oxygenators and metabolise plenty of nitrates present in the water.
  • Marimo are slow growers, at around 5mm per year and with good care can last a century.
  • When the balls do get large you can propogate the plant by tearing it in two and rolling each half into a new ball.
  • Each ball is teeming with chloroplasts which jump into action as soon as the ball either collapses on it’s own weight or is damaged or torn apart so do not worry about it growing back, it is very tough.
  • Usually, snails will not eat marimo but other creatures such as goldfish, crabs and cichlids will likely tear it apart.
  • As an algae, marimo will draw in many nutrients that usual green, hair or brown algae will feed on, effectively limiting other algae growth in an aquarium.
  • They are also excellent to kick start the biological filter in an aquarium as each ball is teeming with bacteria.