A  lagoon is a body of salt or brackish water that's separated from the deeper ocean by a shallow or exposed barrier island, barrier reef, atoll or similar feature. It refers to the back reef habitat where marine plants and organisms are protected from heavy surf and strong ocean currents. The lagoon habitat features soft corals, sponges, gorgonians, anemones, feather worms, various patches of macro algae, juvenile fish and invertebrates. For those that have experienced this environment firsthand, it is difficult to describe the variety and abundance of life that resides there. Some varieties of angelfish, such as the gray and blue angel fish, spend their time as juveniles in these protected shallow waters, feeding on algae, sponge and small invertebrates. 

Macro algae often dominate this environment, including species of Halimeda, Caulerpa, Codium, Neomeris, Padina and Sargassum, as well as other green algae. The habitat is mostly sand and hard bottom, with sporadic growth on small outcroppings of limestone or calcium coralline encrusted rock. Depths here are fairly shallow, ranging from 5 to 30 feet and provide both calm and moderately turbulent waters. Long, shallow tanks are best for this type of biotope, but almost any aquarium will work, if attention is placed on the rock work. Live rock should be used sparingly, just enough for filtration, to form miniature islands, where marine plants and other photosynthetic invertebrates can attach and grow. When planning any biotope or aquascape, it's important to have some inspiration from nature, as the overall effect will often be more aesthetically pleasing. This planted lagoon style (pictured below) aquarium is only 10" deep and is dominated by the species Caulerpa mexicana. Notice the lack of height to the rock work, which is very typical of shallow water lagoon environments. The picture below is the inspiration for the natural aquascape above. Notice the extensive growth of macro algae, in this case various species of Halimeda.