Plants are the most important part of a water microcosm, because all life needs oxygen and plants provide oxygen. Elodea, also called American water weed, is one of the most famous water plants found in aquarium, because the plants is easy to keep, provides good habitat for invertebrate, and is an effective oxygen producer. Elodea is usually the cheapest plant in the aquarium store. Other plants can be used, but the Elodea is easy to cultivate in a microcosm. This plant cost $4.50 at PetSmart. Start the microcosm project by adding the water plant.