Remove the moss ball from the aquarium or other container. Gently squeeze excess water out of the moss ball.
Cut the moss ball in half. Cut each half of the moss ball in half. If the resulting pieces are larger than 2 to 3 inches, cut them each in half. This gives you four to eight new moss balls. Moss balls don't have a kernel or central structure, so it's fine to cut it through the center and in any direction.
Tie thread around each moss-ball cutting in two directions. Wrap the thread around the moss, twist it and wrap it in the opposite direction, the way you'd wrap ribbon in both directions on a gift box. Knot the thread and cut off the excess.
Put the moss ball cuttings in your aquarium or other fresh-water aquatic plant environment. Moss balls prefer low light outdoors or moderate light in an aquarium.
Turn the moss balls at least once a week to encourage growth on all sides of the moss balls. If a marimo moss ball sits in one spot the side that doesn't get any light will die. They also won't become round if they aren't turned regularly because they need light on all sides to grow round.