Azolla caroliniana is a small floating fern housing blue-green algae in its leaves. These algae are capable of absorbing nitrogen from the air, which can then be used by the plant. A. caroliniana has become a menace all over the tropics, because it covers lakes and deprives the original aquatic plants of light. Used as nitrogen fertiliser in rice paddies, and as animal feed. There are several related varieties on the market, with similar appearance. A decorative plant for open aquariums.