Marimo balls (Aegagropila linnaei) are green algae balls that are found in freshwater lakes in the northern hemisphere. They are compatible with most aquatic pets and just need to be submerged in water to live. When they arrive, give them a gentle wash before putting in water. Marimos make perfect plant pets for Children since they are easy to care for. Remember to change the water every 2 weeks to keep the Marimos thriving! If your Marimo balls start getting brown spots, they are getting too much light!