Azolla filiculoides

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.

Water Parameters Azolla filiculoides thrives in soft to moderately hard water across a broad pH range of 5.0–8.0. Keep temperatures between 10–25 °C; it can endure cooler conditions but grows fastest around 20–22 °C. Avoid temperatures above 28 °C for extended periods, as heat stress causes rapid die-off.

Lighting Moderate to bright light (50–150+ µmol) intensifies the natural red coloration. Under low light the foliage stays green and growth slows but the plant remains healthy. Avoid completely shading it with other floating plants.

Nutrition Azolla's symbiotic cyanobacteria fix atmospheric nitrogen, so it rarely needs nitrogen supplementation. However, it benefits from phosphate and micronutrients in the water column. In heavily planted tanks with CO2 injection it will consume nutrients rapidly and may require additional trace elements.

Maintenance This plant grows so quickly that weekly thinning is essential. Remove roughly half the mat every 5–7 days to maintain light penetration for submerged plants and to prevent oxygen depletion near the substrate. Collect removed portions and compost them; they are excellent fertilizers for terrestrial plants.

Placement and Co-inhabitants Leave a gap between the water surface and any cover glass or hood to ensure good gas exchange and access to atmospheric nitrogen. Bettas, gouramis, and killifish that build bubble nests appreciate the surface cover. Avoid strong surface agitation, which breaks apart the delicate fronds.

Common Problems Melting or browning usually indicates water temperatures above 28 °C, excessive salt, or sudden parameter shifts. Algae growing on the fronds signals poor water quality or insufficient flow beneath the mat.