Salvinia (Salvinia natans / minima)
If your new planted tank is dumping ammonia faster than the slow rooted plants can mop it up, Salvinia is the classic first line of defence. It is a small free-floating fern native to slow-moving freshwater across Europe, Africa, and Asia, and a staple of low-tech aquariums. Its fronds are oval pads a centimetre or two across, dressed in water-repellent hairs that keep them riding on the surface; a third, thread-like submerged leaf hangs below and does most of the nutrient-gathering work, behaving like a tuft of roots (Lemon & Posluszny 2000). In the model, Salvinia is the primary tool for taming the early ammonium spike of a Walstad-style tank — exactly the role Walstad (1999, ch. 5) recommends it for.
A fast grower that lives in full light
Floating at the surface, Salvinia sits in the brightest, most CO₂-rich spot in the tank: atmospheric carbon dioxide diffuses straight into the fronds, so carbon is rarely what limits it. Given that light and a supply of nutrients, it grows quickly — a doubling time of roughly four days in moderate light, closer to three in bright light, matching the two-to-five-day range reported for non-invasive populations (Lemon & Posluszny 2000; Tanner & Headley 2011). It is a warm-water plant, happiest near 26 °C, stressed by cold, and killed outright by frost. It is strictly freshwater and tolerates a wide span of pH, reflecting the broad natural range of its habitats.
Its tissue is protein-rich and carries no woody lignin, which is part of why it grows fast and why, when fronds die, they sink rather than linger — the great majority of dead frond mass settles to the bottom rather than clouding the water.
The ammonia sponge
Salvinia's value in a young tank is its appetite for dissolved ammonium. It carries a high-affinity uptake system — the same kind characterised for duckweed (Cedergreen & Madsen 2002) — that runs at close to full speed even when ammonium is dilute. As soon as a spiked substrate starts releasing ammonia faster than the rooted plants can absorb it, a small floating cluster intercepts it from the water column. In the 180-day Walstad test scenario, introducing a pinch of Salvinia on day 3 cut the day-180 water-column ammonia roughly threefold (from about 1.8 down to 0.6 mg N/L) — the difference between a tank that reads "hot" on a test kit for months and one that settles quickly.
A living lid
As Salvinia spreads it becomes a canopy, and that canopy is double-edged. On the helpful side, a dense mat blocks the great majority of the light reaching the water below — well over 95% at full coverage — which starves competing algae of the light they would otherwise bloom on. The shading follows the usual Beer-Lambert law: each layer of frond intercepts a share of the light, so a thickening mat dims the column steeply, and the fronds also self-shade one another, which softly throttles the mat's own growth as it crowds. On top of that soft limit there is a hard one — the mat can cover only so much of the surface (around 95% for Salvinia) before there is simply no open water left to colonise.
The less helpful side is gas exchange. A floating mat is a physical cap on the water surface, and it quietly suppresses the wind- and convection-driven turbulence that lets the water breathe — oxygen in, carbon dioxide out. Salvinia is gentler about this than tight-packed duckweed: its water-repellent hairs lift the fronds slightly clear of the surface and the mat stays relatively porous, so a Salvinia-choked tank still exchanges gas at perhaps a quarter to a third of open-water rates (Janes 1998; Mitchell & Tur 1975). Enough to matter on a still night, but far less suffocating than a duckweed lid.
Salvinia versus duckweed at a glance
Salvinia and duckweed are the two floating plants in the model, and they make a natural pairing — both scavenge ammonia, both shade the water, but they live at different tempos:
| Trait | Salvinia (this page) | Duckweed |
|---|---|---|
| Frond size | Centimetre-scale oval pads | Millimetre-scale discs |
| Growth pace | Fast — doubles in about four days | Faster — doubles in about two days |
| Ammonia scavenging | Strong | Strongest of any floating plant |
| Shading per unit mass | High | Higher — tiny fronds pack light-blocking tightly |
| Gas-exchange blocking | Moderate (porous, hair-lifted mat) | Severe (a near-continuous lid) |
| In a mixed mat | Slowly pushed under by duckweed | Wins the surface over weeks to months |
The exact growth rates, light and ammonia half-saturations, temperature and pH thresholds, and canopy coefficients behind this contrast are tabulated in the Parameter Reference.
Further reading
- Duckweed — the smaller, faster floating cousin that eventually crowds Salvinia off the surface
- Macrophytes: Aquatic Plants — how floating, rooted, and submerged plants differ, and the floating-mat mechanics in full
- Producers — how all the algae and plants fit together
- Nitrogen Cycle — the ammonia spike Salvinia is brought in to control
- Parameter Reference — every rate, half-saturation, and threshold behind this page, with citations
Key references
- Cedergreen, N. & Madsen, T.V. (2002). Nitrogen uptake by the floating macrophyte Lemna minor. New Phytologist 155, 285–292.
- Janes, R. (1998). Growth and survival of Azolla filiculoides in Britain. New Phytologist 138, 367–375.
- Lemon, G.D. & Posluszny, U. (2000). Comparative shoot development and evolution in the Lemnaceae. International Journal of Plant Sciences 161, 733–748.
- Mitchell, D.S. & Tur, N.M. (1975). The rate of growth of Salvinia molesta in laboratory and natural conditions. Journal of Applied Ecology 12, 213–225.
- Tanner, C.C. & Headley, T.R. (2011). Components of floating treatment wetlands influencing removal of stormwater pollutants. Ecological Engineering 37, 501–509.
- Walstad, D.L. (1999). Ecology of the Planted Aquarium. Echinodorus Publishing.