Amazon Frogbit Care Guide

Amazon frogbit (Limnobium laevigatum) is a South American floater with round, slightly cupped leaves that sit flat on the surface and a long, pale root system hanging beneath them. In a mature tank those roots reach fifteen to twenty-five centimetres, and they are the reason to keep it: shrimp graze the biofilm on them all day, and fry treat the curtain as a nursery.

Most floaters are bought and then resented for shading the tank. Frogbit earns its shade differently — the root mass is functional habitat, not a side effect, and the nutrient uptake that comes with it is enough to visibly drag nitrate down in a lightly filtered tank.

It is not fussy about water chemistry, light or CO2. The one thing it will not tolerate is water landing on the tops of the leaves, which is a mechanical problem and not a care problem.

A dozen rosettes is plenty. Frogbit multiplies by runners and a small starter portion covers a two-foot tank within a month under any usable light.

Snail eggs and duckweed travel with it more often than with any other plant on this site, because both hide among the roots. Rinse the portion in a bucket and pick through the root mass before it goes in the tank.

Low light is enough. Frogbit sits directly under the fixture with no water column absorbing anything, so even a weak LED delivers more to it than to a plant on the substrate.

Brighter light makes it grow faster and produce more runners, which in practice means more thinning rather than a better-looking plant. The leaves stay green either way — there is no colour response to chase here.

The plant to think about is whatever is underneath. A full frogbit mat can cut light to the substrate dramatically, and that is a bigger lighting decision than anything the frogbit itself needs.

Not needed. Floating plants take carbon dioxide from the air through their upper leaf surfaces, which is a far richer source than anything dissolved in the water.

Injecting CO2 for a frogbit mat is wasted gas. It benefits the submersed plants below and does close to nothing for the floater above them.

All feeding happens through the roots hanging in the water column, so root tabs are pointless. A standard all-in-one liquid fertiliser covers it, and in a well-stocked tank fish waste often covers it outright.

Iron shortage is the usual complaint. Frogbit that goes pale yellow between the leaf veins while continuing to push out new rosettes is short of iron, not light — dose a chelated iron supplement and the colour returns within a fortnight.

The plant is genuinely good at nutrient export. Pulling out a handful every week removes the nitrogen and phosphorus locked in it, which is a real and often overlooked part of algae control in a low-tech tank.

Water chemistry barely registers. Anything from pH 6.0 to 7.5, 2 to 15 dGH and 20 to 28 °C suits it, and it tolerates the swings of an unheated room tank better than most tropical plants.

Surface conditions are the real parameter. Frogbit needs a calm top and, just as importantly, a gap between the water and the lid — a leaf pressed against warm cover glass cooks and rots.

The decision is not where it sits but how much surface it gets. Left alone, frogbit takes the lot in about a month.

A third to a half is the useful range. That gives fish an open area to breathe and feed in, keeps enough light reaching the substrate for the plants below, and still leaves a proper root curtain for shrimp and fry.

A barrier keeps it honest. Suction-cupped airline, a floating ring, or a strip of rigid tubing across one end all hold the mat away from the filter outflow and stop it drifting over the whole tank.

Growth is fast: a rosette throws a runner, the daughter plant matures, and both throw more. Doubling every two to three weeks under decent light is normal.

Maintenance is scooping, not trimming. Take out whole rosettes rather than cutting leaves, and take them from the crowded middle of the mat where the plants are stacking on top of each other.

Watch the leaf undersides on plants near the outflow. Once the spongy layer under the leaf becomes waterlogged the rosette will not recover, and one rotting plant in the middle of a tight mat fouls the ones around it.

There is nothing to do. Frogbit sends out short stolons with a daughter rosette at the end; the connection breaks on its own once the daughter is established.

Moving it between tanks is instant — float it and it carries on. The only rule is not to crush the roots when you lift it, because a bruised root mass sheds and takes a few weeks to rebuild.

Limnobium laevigatum grows on still, nutrient-rich water across tropical and subtropical America, in the kind of sun-exposed pond margin where nothing disturbs the surface. It is naturalised well beyond that range, and it appears on invasive-species watch lists in several countries even where it is not formally restricted.

The long roots are a response to competition rather than to poverty: on a crowded surface the plant that reaches deepest into the water column gets the nutrients. That is exactly why they grow so long in a clean, lightly stocked aquarium and stay short in a heavily fed one.

The spongy tissue under each leaf is a flotation structure, and it explains the single care rule on this page. It works by staying dry. Waterlog it and the rosette sits lower, takes on more water, and rots.