Red Root Floater (Phyllanthus fluitans)

Red root floater is the floating plant people grow for its looks. It is a small free-floating flowering plant from the still and slow-moving waters of tropical South America, and in the aquarium trade it is prized for one thing above all: under strong light it flushes a deep crimson — first the fine roots that trail beneath it, then the undersides and edges of its rounded leaves, and eventually whole rosettes of burgundy riding on the surface. Each plant is a little cluster of kidney-shaped pads about a centimetre across, kept afloat by a felt of water-repellent hairs on the upper surface, with a conspicuous tuft of long, thread-like roots hanging into the water below. It fills the same ecological niche as Salvinia and duckweed — surface shade and water-column nutrient uptake — but with a very different temperament.

A moderate grower that demands light

Where duckweed and Salvinia will blanket even a dim surface, red root floater hangs back. It is the slowest of the three floaters in the model and the most light-hungry: in a low-light tank it stays small, sparse, and plain green, and it only really fills in — and only colours up red — under strong light. Given that light and a supply of nutrients it doubles in roughly three weeks, appreciably slower than Salvinia (a week or so under the same conditions) and far slower than duckweed (a matter of days). Like all floating plants it sits in the brightest, most CO₂-rich spot in the tank and takes carbon dioxide straight from the air through the exposed upper surface of its leaves, so carbon is rarely what limits it — light is.

The red colour itself is a sunscreen. The pigments (anthocyanins) that the leaves and roots accumulate under intense light are a photoprotective response, shielding the photosynthetic machinery from more light than it can use (Steyn et al. 2002; Close & Beadle 2003); they fade back to green in shade or under heavy nitrogen loading. The simulator tracks only the plant's carbon, nitrogen, and phosphorus, not its pigments, so in the tank view the reddening is drawn as a fixed part of the plant's look rather than something that comes and goes with the lighting — but the underlying biology is a high-light stress response, not a sign of health.

A tropical plant that resents the cold

Red root floater is strictly warm-water. It is happiest between about 24 and 28 °C, sulks below roughly 18 °C, and is killed by cold well before Salvinia — and long before the cold-hardy, turion-forming duckweed — would be troubled. It prefers soft, slightly acidic to neutral water, and like every floater it dislikes surface disturbance and trapped condensation: wetted leaves rot. It is a freshwater plant with no tolerance for salt.

A capable but unaggressive nutrient sponge

Its trailing roots and leaf undersides carry the same kind of high-affinity uptake system that makes small floating plants such good ammonium scavengers (the mechanism characterised directly for duckweed by Cedergreen & Madsen 2002). Red root floater draws nitrogen and phosphorus from the water column and will help mop up a nutrient surplus, but it is a less avid and slower feeder than either Salvinia or duckweed, in keeping with its more modest growth rate. It is a contributor to nutrient control, not the frontline tool that a young Walstad-style tank leans on to tame its early ammonia spike.

First to be shaded out of a mixed mat

Put red root floater on the same surface as duckweed or Salvinia and, over weeks, it is the one that loses. Because it is the slowest grower in the floating community, once the combined mat gets dense it accumulates the extra mortality of being crowded and pushed under into the dark, while the faster plants take no such penalty — the same displacement mechanism that lets duckweed overrun Salvinia, now acting on red root floater from both sides. In a mixed floating tank it survives best where the faster floaters are kept trimmed back and it has open, brightly-lit surface to itself.

The three floaters at a glance

The exact growth rates, light and nutrient half-saturations, temperature and pH thresholds, and canopy coefficients behind this contrast are tabulated in the Parameter Reference.

A note on the data

Unlike duckweed and Salvinia — both weeds of economic importance with a large research literature — Phyllanthus fluitans is an ornamental with very little published ecophysiology. Its parameters in the model are therefore set by analogy to the well-studied floaters and then adjusted to reproduce the three things aquarists consistently report: a moderate, sub-Salvinia growth rate, a high light requirement, and a low tolerance for cold. The values are a defensible best estimate, not a direct measurement, and are flagged as such in the parameter reference.

Further reading

• Salvinia and Duckweed — the two faster floaters that shade red root floater out of a shared surface
• Macrophytes: Aquatic Plants — floating-mat mechanics, including how a mat throttles gas exchange and shades the water below
• Producers — how all the algae and plants fit together
• Nitrogen Cycle — the water-column nutrients floating plants draw on
• Parameter Reference — every rate, half-saturation, and threshold behind this page, with citations

Key references

• Close, D.C. & Beadle, C.L. (2003). The ecophysiology of foliar anthocyanin. Botanical Review 69, 149–161.
• Cedergreen, N. & Madsen, T.V. (2002). Nitrogen uptake by the floating macrophyte Lemna minor. New Phytologist 155, 285–292.
• Cook, C.D.K. (1990). Aquatic Plant Book. SPB Academic Publishing.
• Lemon, G.D. & Posluszny, U. (2000). Comparative shoot development and evolution in the Lemnaceae. International Journal of Plant Sciences 161, 733–748.
• Steyn, W.J., Wand, S.J.E., Holcroft, D.M. & Jacobs, G. (2002). Anthocyanins in vegetative tissues: a proposed unified function in photoprotection. New Phytologist 155, 349–361.
• Walstad, D.L. (1999). Ecology of the Planted Aquarium. Echinodorus Publishing.