Azolla Care Guide

For European readers the first fact is a legal one: Azolla filiculoides is on the EU list of regulated invasive species, and keeping, selling, transporting or releasing it is prohibited across all member states. The UK has banned its sale since 2014. If you are in either jurisdiction, this page is background reading rather than a shopping list, and the disposal guidance above matters more than the care advice below.

The biology behind that listing is also what makes azolla interesting. It is the only plant in this database that fixes its own nitrogen: each frond houses a symbiotic cyanobacterium in a small cavity, and that partner converts atmospheric nitrogen into a form the fern can use. Azolla therefore grows explosively in water that contains almost no nitrate — the condition that limits every other plant here.

That capability is why azolla has been used as a rice paddy fertiliser in Asia for centuries, long before anyone put it in an aquarium, and it is why a small starter portion covers a surface in weeks rather than months.

Physically it is tiny. Individual fronds are two to five millimetres across, overlapping into a fine-textured mat that turns deep red under strong light or cold conditions.

Colour on arrival tells you about the light it came from, not the plant's health. Green stock reddens within weeks under a bright fixture, and red stock greens up under a dim one.

Take nothing from a wild pond, ditch or canal. Beyond the legal exposure in restricted regions, wild azolla arrives with the full complement of pond invertebrates and pathogens.

Medium light is the target and it is doing two jobs: driving growth, and driving colour. Azolla under a strong fixture turns from green through bronze to a deep red across the whole mat, and that colouration is the main aesthetic reason to keep it.

Cold also reddens it, so a mat in a cool, brightly lit tank is at its most colourful. In a dim, warm tank it stays plain green.

What sits below it is the real consideration. A closed azolla mat is dense and blocks a great deal of light from the tank underneath — thinning it is a lighting decision for the plants below as much as a tidiness one.

Not needed. Like every floater it takes carbon dioxide from the air across its upper surface, which is far richer than anything in solution.

CO2 injection does nothing for it and is not wasted only because the submersed plants below will use it.

This is the section where azolla behaves unlike anything else here. Its symbiotic cyanobacterium fixes atmospheric nitrogen inside the frond, so the plant is not dependent on nitrate in the water at all. In a lean, well-maintained tank where other plants stall for want of nitrogen, azolla keeps doubling.

It still needs everything else from the water — phosphorus, potassium, iron and trace elements — and in a very lightly stocked tank those are what limit it. Phosphorus shortage is the usual brake, and it shows as a mat that stops expanding while staying healthy in colour.

A consequence worth understanding: azolla is a poor nitrate remover compared with frogbit or hornwort, because it is not obliged to take nitrogen from the water. Do not add it expecting the nitrate control that other floaters provide.

Iron shortage shows as pale, yellowish fronds and a mat that will not redden no matter how bright the light.

Broadly tolerant across pH and hardness, and comfortable well below tropical temperatures — it survives near-freezing conditions outdoors and overwinters in temperate ponds, which is a large part of why it is invasive in Europe.

A calm surface is what it needs most. Strong surface agitation pushes the mat into corners, waterlogs fronds and eventually sinks them.

Very warm water is the one condition it is less fond of: above about 28 °C growth slows and the mat thins out.

The surface, and as much or as little of it as you allow. Azolla has no other placement.

Confining it is worth doing from day one. A feeding ring, an airline tied across a corner, or a cut length of tubing floated as a boom will hold the mat in one area while the rest of the surface stays open.

Leave open water for surface breathers. Anabantoids and corydoras need somewhere to reach the air without pushing through a mat, and a fully closed surface makes that harder than it looks.

Thinning is the whole maintenance routine and it is constant. In good conditions the mat roughly doubles within a week, so a session with a fine net every water change is the minimum.

It is easier to remove than duckweed, which is worth saying plainly: the fronds are larger, they clump together rather than dispersing individually, and a net takes out most of a mat in one pass.

The comparison people are really making is with the other small floaters. Salvinia is the closest relative — also a floating fern, also EU-regulated — but the two are not interchangeable. Salvinia has larger, water-repellent leaves covered in fine hairs that shed splash, which makes it the one to choose in a tank with surface movement. Azolla is smaller, denser and much less tolerant of agitation, and it brings nitrogen fixation and stronger red colour that salvinia does not. Duckweed is the third option and the one to avoid: individual fronds are almost impossible to catch and it is notoriously permanent.

A mat left to thicken stacks into layers, and the fronds underneath rot for want of light. Thin by area rather than skimming the top.

There is nothing to do. Azolla branches and the branches separate on their own, which is the entire propagation mechanism and the reason a pinch becomes a mat.

It also produces spores, though in aquarium conditions vegetative division is so rapid that spores are irrelevant to how the population grows in your tank.

Azolla filiculoides is native to the Americas and naturalised across Europe, Africa, Asia and Australasia, where it forms mats thick enough to cover an entire pond surface. That capacity for total cover — deoxygenating the water beneath and shading out everything in it — is what put it on the EU list rather than any effect on aquariums.

The nitrogen fixation is the reason it can do that. The symbiotic cyanobacterium living inside each frond supplies nitrogen that the surrounding water does not, so azolla colonises clean water where competing plants are nitrogen-limited. It is a genuine evolutionary advantage and a genuine management problem at the same time.

That same trait made it an agricultural crop. Azolla has been grown in rice paddies across China and Vietnam for centuries as a green fertiliser, and it is still farmed as livestock feed — which is why searches for the plant return far more agronomy than aquarium keeping.

Cold tolerance completes the picture. Azolla overwinters in temperate climates, which is why an aquarium portion tipped into a garden pond in a cool country does not simply die off.