Azolla filiculoides (Red Water Fern)

Azolla filiculoides is a remarkable miniature floating fern that harbours a symbiotic nitrogen-fixing cyanobacterium (Anabaena azollae) within its leaf cavities, enabling it to fix atmospheric nitrogen and thrive in nutrient-poor water. Under bright light the fronds develop vivid red to burgundy pigmentation, turning entire water surfaces into a shimmering ruby-red carpet. In lower light it remains green. This dramatic colour-shifting quality makes it one of the most visually dynamic surface plants available for paludariums.

Beyond aesthetics, Azolla is a powerful water purifier, absorbing nitrogen, phosphorus, and heavy metals from the water column with exceptional efficiency. It has historically been used as green manure in rice cultivation for this reason. In paludariums it provides dense surface cover that diffuses light, reduces algae growth, and cools the water surface — beneficial qualities for shade-preferring inhabitants.

Light: Medium to high; 2,500–8,000 lux; red colouration requires strong light; green in shade. CO2/Humidity: No CO2; avoid turbulence that submerges fronds; humidity above the water benefits coverage density. Fertilisation: No fertilisation needed; fixes its own nitrogen; absorbs other nutrients from water. Placement: Float freely on the water surface; harvest excess regularly as it doubles in volume every 3–5 days under ideal conditions. Propagation: Divides naturally and prolifically by fragmentation; needs no assistance.