A "biotope aquarium" is an aquarium that is designed to simulate a natural habitat, with the fish, plants, and furnishings all representative of a particular place in nature. Because only species that are found together in nature are allowed in a true biotope aquarium, these tanks are more challenging and less common than the other themes. The most common biotope aquariums are the Amazon biotope and the Lake Malawi biotope, but occasionally aquarists will recreate the South East Asia river biotope.