Alternanthera aquatica

Native to South American wetlands, this robust plant is characterized by hollow, buoyant stems allowing it to thrive as a floating mat or a rooted aquatic vine. Widely recognized in the aquarium trade for its impressive size and vigorous growth habits, this species functions excellently as an amphibious flora suitable for open-topped tanks, ripariums, and paludariums. Unlike many delicate aquatic plants, this species possesses a hardy constitution that allows it to bridge the gap between the water column and the air, utilizing its specialized stems to float along the surface while sending long root systems down toward the substrate. These extensive roots act as natural water purifiers, aggressively absorbing excess nutrients and nitrates from the water column, which contributes significantly to maintaining a healthy ecosystem for fish and invertebrates. While it naturally seeks direct access to the atmosphere, it can be maintained in a submerged state with appropriate care, though it will constantly strive to reach the surface. Due to its substantial mature size, it is best reserved for larger setups where it can spread without crowding out smaller tank mates. Owners should expect rapid development when environmental conditions are met, necessitating regular maintenance to keep its expansive growth in check. Its ability to create shaded areas below the surface provides excellent shelter for shy livestock, making it a functional addition to community tanks simulating tropical river environments.

First described in South American botanical surveys, this member of the Amaranthaceae family presents unique challenges and rewards for the dedicated aquarist. It is strictly an amphibious organism, evolved to survive the fluctuating water levels of its native tropical wetlands. The plant distinguishes itself through a physiological adaptation known as aerenchyma tissue—large air-filled spaces within the stems—which provides the buoyancy required to form extensive floating mats in the wild. In an aquarium setting, this dictates the plant’s behavior; even when planted in the substrate, the stems will rapidly elongate toward the water surface to access atmospheric carbon dioxide. Consequently, this species is considered a heavy feeder with a high metabolic rate, requiring a nutrient-rich substrate composed of soil, clay, or specialized aquarium sands to fuel its rapid expansion. Lighting plays a critical role in the plant's morphology. To maintain compact aquatic growth, high-intensity illumination is required. Under lower light conditions, the plant tends to become 'leggy,' shedding lower leaves and stretching its internodes unappealingly as it desperately seeks a light source. While it can survive without supplemental carbon dioxide injection, the addition of CO2 greatly enhances its density and vitality when kept fully submerged. However, the most robust growth is invariably observed when the plant is allowed to breach the surface, transitioning into its emersed form. In this state, its demand for water column nutrients increases, turning the plant into an effective biological filter that significantly reduces bioload. Stability in water chemistry is preferred, though the plant is adaptable to a range of hardness and pH values typical of tropical freshwater systems. It thrives in warmer thermal ranges, reflecting its tropical origins, and growth may stall significantly if temperatures drop too low. For maintenance, aquarists must be proactive; the plant’s aggressive growth can easily shadow underlying vegetation, depriving bottom-dwelling flora of light. Regular pruning is not only cosmetic but essential for the health of the entire tank ecosystem. When trimming, the hollow nature of the stems becomes apparent, and care should be taken not to crush the vascular tissues, which can lead to rot.