Guppies are great companion for Marimo as they help to keep ugly bugs/worms such as mosquito larvae away from your Marimo. Moreover, guppies are hardy fish that do not need aeration as long as you only keep few of them in your Marimo container. However, guppies could not survive in chlorinated water; you will need to treat the water with anti-chlorine which can be bought cheaply in most fish stores.
