Marimo moss ball

Overview

Despite the name, a marimo moss ball is not a moss but a green alga (Aegagropila linnaei). In a handful of cold lakes in the Northern Hemisphere, such as Lake Akan in Japan and lakes in Iceland and northern Europe, gentle water movement rolls it into velvety spheres on the lake bed.

The ones sold in stores are 1-2.5 inches (2-6 cm) across and are usually cultivated or hand-rolled. People love them for their quirky shape, because they need almost no care and because they live for many years. They suit unheated tanks, tropical tanks that are not too warm and shrimp tanks, where they double as a grazing surface.

Care and tank setup

Their weak spot is heat: they do best below 75 °F (24 °C). Kept constantly at 79-82 °F (26-28 °C), they tend to turn brown and fall apart. They also dislike direct sunlight.

• Light: low; a standard aquarium light or indirect daylight is enough.
• Roll them: turn them over now and then so every side gets light and they keep their round shape.
• Cleaning: at each water change, gently squeeze them in tank water to release trapped debris.
• Tankmates: fine with shrimp, snails and peaceful fish; goldfish and large fish may tear them apart.

Buy only from trusted sellers and inspect them on arrival: in 2021, invasive zebra mussels were found in marimo shipments sold in North America. Never release them into ponds, lakes or streams.

Fertilizing and trimming

They need no fertilizer. They take nutrients from the water and grow extremely slowly, only a few millimeters a year. Brown patches usually point to heat, too little light or trapped dirt: rinse the ball and move it somewhere cooler. If part of it has rotted, trim it off with clean scissors. Avoid algaecides, which can kill them.

Propagation

Marimo propagate by division: cut a ball into two or more pieces with clean scissors, roll each piece in your hands to shape it, and wrap it with cotton thread so it holds together. Over time the filaments form a sphere again. A large ball sometimes buds off small daughter balls you can separate.

Recommended equipment

What you need to keep the Marimo moss ball healthy, checked against Amazon stock.