Plants occupy a critical niche in any natural ecosystem helping to remove waste compounds that are produced but not consumed by other living organisms. Marine aquarist have appreciated the benefits to water quality when actively growing plants are a part of their tanks since the 1970's.
The first plants commonly added were species of Caulerpa. Initially their primary purposes were to create a more natural look and provide something for the inhabitants to graze on but the positive effect they had on nitrate and phosphate management were soon realized.
Marine aquarium plants are usually divided into two main categories the macro algae and sea grasses. The difference between these two is their structure. The sea grasses have a traditional root and leave structure with separate and distinct functions comparable to many of the plants found on land. Macro algae on the other hand conduct these same functions on a cellular level. The algae sometimes take physical forms that are similar to more complex plants because these serve to maximize exposure to nutrients in the current and available light.
The macro algae fall into three categories green algae (Chlorophyta), red algae (Rhodophyta) and brown algae (Phaeophyta). The sea grasses belong to the phylum Magnoliophyta also known as angiosperms that are flowering plants. Some of the more popular aquarium sea grasses include several Halophila species that tend to have wider more oval shaped leaves and a more traditional plant shape as opposed to the Thalassia, Zostera and Syringodium which have more grass like ribbon shaped leaves.
Along with using waste products as food these plants also consume minerals to build tissue particularly calcium. They require the same nutrients as land based plants so water testing and fertilization is needed for proper growth.