Azolla filiculoides, commonly known as Fairy Moss or Red Water Fern, is a small aquatic fern native to the Americas but now naturalized across temperate and tropical regions worldwide. It is not a true moss but a floating fern with overlapping scale-like leaves arranged in two rows along branching stems, giving it a feathery, moss-like appearance. Under high light conditions the foliage turns striking shades of red and burgundy, while lower light keeps it bright green. The plant hosts the nitrogen-fixing cyanobacterium Anabaena azollae in specialized cavities within its leaves, allowing it to enrich the water with bioavailable nitrogen — a trait that has made it famous as a rice-paddy fertilizer for centuries. In the aquarium it grows extremely quickly, doubling its mass in days under favorable conditions, and must be thinned regularly to prevent it from blocking all light to submerged plants. It provides excellent surface cover, reduces algae by outcompeting it for nutrients, and offers dense shelter for shrimp, fry, and labyrinth fish that appreciate surface vegetation. Azolla tolerates a remarkably wide pH and temperature range, thriving from cool temperate to warm tropical tanks. In strong light and nutrient-rich water it flushes deep red, creating a dramatic contrast above green aquascape layouts. It is one of the easiest floating plants available and an ideal starter plant for any aquarium.
