Floating Plants in Aquariums
Salvinia, Pistia, frogbit, duckweed and red root floater: how floating plants take up nutrients and air CO2, how they shade the tank, how to thin them and why several are invasive.
What floating plants are
Free-floating plants rest on the water surface with roots that are not anchored in the substrate, so they drift with currents and wind. Lightweight, air-filled tissue (aerenchyma) keeps them afloat. Their leaves carry stomata on the upper surface, open to the air, which gives them a direct supply of atmospheric carbon dioxide. Submerged plants, by contrast, are limited by the slow diffusion of CO2 through the boundary layer of water around their leaves.
Common aquarium species
- Salvinia (floating ferns): each node bears two flat floating leaves and a third submerged, finely divided leaf that looks like roots; the plants have no true roots. Water-repellent hairs cover the upper leaf surface. Species seen in aquaria include S. natans, S. minima, S. auriculata and S. cucullata.
- Pistia stratiotes (water lettuce): rosettes of thick, soft leaves with long dangling roots, spreading mainly by stolons. It grows best at about 22–30 °C and does not grow in cold climates outside the tropics.
- Hydrocharis laevigata, long known as Limnobium laevigatum (Amazon frogbit or West Indian spongeplant): native to tropical and subtropical Mexico, Central America, the Caribbean and South America. Young plants have spongy tissue under the leaves for flotation; mature plants can raise emergent leaves to about 50 cm. It spreads by seed and by fragments of stolons.
- Lemna and other duckweeds (Lemnoideae): tiny floating plants that can double their biomass in about four and a half days.
- Phyllanthus fluitans (red root floater): an Amazon-basin species with heart-shaped leaves holding air pockets along the midrib, red roots and leaves that turn reddish under bright light.
Nutrient uptake
Because they are not limited by CO2, floating plants can grow quickly close to the light, and most aquarium species consume a large share of the nutrients in the water. Duckweeds absorb nitrogen and phosphate efficiently enough to be used in wastewater treatment, and Pistia is used to strip excess nutrients and heavy metals such as zinc, chromium and cadmium from contaminated water. In an aquarium this nitrogen uptake competes with algae. Floating plants depend on nutrients dissolved in the water column, unlike rooted plants with access to the substrate, so in a lean tank they may show yellowing leaves and stunted growth.
Competition also works in the other direction. In co-culture experiments, the free-floating Lemna gibba was held back by dense stands of the rootless submerged Ceratophyllum demersum, mainly through raised pH and depletion of phosphorus, nitrogen and iron.
Shading effects
A floating cover shades everything beneath it. In natural small, still waters, dominance of free-floating plants suppresses submerged plants by shading, and dense mats of Salvinia molesta or Pistia block light and gas exchange at the surface, which can cause severe oxygen depletion. In an aquarium the same effect reduces light reaching stem and carpet plants below and also dampens surface movement. Dim, sheltered surface zones suit fish such as Betta, and the fine root curtains give cover and spawning sites to fry, shrimp and small fish; tetras have been observed spawning among them.
Control and containment
- Thin the cover regularly, removing whole plants rather than cutting them, especially when submerged plants show signs of too little light.
- Long roots can be trimmed with scissors if they hang too far into the tank.
- Water lettuce and red root floater in particular tolerate very little water movement; strong surface flow damages and scatters them, so they do best in calm areas of the surface.
- Fast-growing duckweeds, Salvinia and the floating liverwort Riccia fluitans regenerate from tiny fragments, so complete removal from a tank is difficult once they are established.
Giant salvinia was spread deliberately as an aquarium and water-garden plant and accidentally as a contaminant in shipments of other aquatic plants. Excess floating plants should never be released into ponds, rivers or drains. Alternatives are returning them to a retailer, giving them to another aquarist, pond keeper or aquarium society, or sealing them in plastic bags and putting them in household waste. They should not be composted, because plant fragments and seeds can survive and spread.