The marimo is the friendliest oddity in the aquarium trade: a soft green velvet ball that asks almost nothing and can outlive the keeper. The first thing to know is that it is not a moss, despite the name stuck on it everywhere.
What a marimo actually is
A marimo is a green alga, Aegagropila linnaei, that in a few cold, clean lakes in Japan, Iceland and northern Europe grows into dense spheres rolled smooth by gentle currents. The ball is not a single organism wrapped round a core but a colony of filaments growing radially, green all the way through. Wild marimo are slow-growing and rare enough to be legally protected in their native lakes, so the ones sold are cultivated or hand-rolled from loose filaments rather than taken from the wild.
Not a moss, and why the name stuck
The English name is a straight translation of the Japanese, where mari is a ball and mo a general word for water plants, so the ball part is fair and only the moss part misleads. It is easy to see how it caught on: the thing is green, velvety and undemanding, and it sits in a tank behaving much as a clump of aquarium moss would. But a marimo has none of the parts that make a moss a moss, no tiny leaves, no stems, no rhizoids gripping a surface. It is a single kind of filamentous alga wound into a ball, and knowing that explains its care, since it wants the clean, cool, gently moving water of a lake rather than anything a land moss would ask for.
How to keep one
Marimo want the conditions of the cold lakes they come from: cool water, gentle or no flow, and modest light. They are happiest below about 22 degrees and dislike a hot, brightly lit tank, which is one of the few ways to actually harm them. Keep them in clean, cool water, give them an occasional swill in old tank water or dechlorinated water to rinse out trapped debris, and that is more or less the whole regime. Looked after, they live for decades; some kept specimens are said to be over a century old.
Keeping it round and green
Two simple habits keep a marimo looking right. Roll it gently in your hand now and again, or rely on a little water movement to turn it, so every side gets light and it stays spherical rather than flattening where it rests. And keep it out of strong light and warmth, which bleach it pale or let it brown and fur with other algae. If a ball goes patchy, a rinse, a gentle squeeze and a spell somewhere cool and dim usually brings it back; a badly misshapen one can be rolled back into form by hand, or pulled apart and re-rolled from the healthy green filaments if the centre has gone.
When something goes wrong
Most marimo trouble shows in colour and shape. Brown or black patches usually mean trapped debris or a spot that never sees light, and a rinse, a squeeze and a turn onto a fresh side generally sort them. A ball that goes pale and yellowish is getting too much light or warmth, so move it somewhere cooler and dimmer. Brown or grey fuzz on the surface is other algae taking hold, often a sign the tank is too bright. White fluff, by contrast, is worth treating seriously and isolating the ball at once, since it can point to a growth that spreads to healthy stock. None of this is common, which is part of why the marimo has such a mild reputation.
In the tank, and with shrimp
Marimo sit happily in a community tank and are a particular favourite in shrimp tanks, where the residents graze the biofilm off the surface and pick through the filaments for scraps. They are soft, harmless and trap no sharp edges. Because they are an alga rather than a rooted plant, you simply set them on the substrate or wedge them where you like. For the wider untangling of which aquarium "mosses" are the real thing, see which aquarium mosses are really moss.