Floating plants

Floating Plants. Ceratopteris cornuta (Water Sprite) is a floating fern that's so fast-growing and undemanding that it gains a place on even a rank beginner's list. Its leaves and rootlets stabilize the bubble nests of labyrinth fishes. Adventitious plants form around the margins of the floating leaves, so you keep plucking them away and discarding the old mother plant,  eliminating as you do so all the nitrogen that is locked up in its structure, which you're exporting from your nitrogen cycle. All to the good. 
 
When  its hanging rootlets  take hold in moist soil, the underwater leaves of the rooted plant take such a different form that many botanists consider it a different species, Ceratopteris thalictroides, "with the leaf like Thalictrum, Meadow-rue". C. cornuta has such a wide distribution, from Africa through India, Burma and Indonesia to tropical northern Australia, that some variation in leaf structures might be expected; botanists who split Ceratopteris do note many "hybrids" in overlapping populations. My hunch is that we're seeing an overlapping range of populations within the one species: in C. thalictroides sampled from various locations, the chromosome count has been variously recorded, and n = 40 has been attributed on different occasions to specimens identified by their form both as C. thalictroides and C. cornuta. 
 
Lemna minor (Duckweed). The minute oval leaves of duckweed each carry a single root, which serves like a weighted keel to set the leaf upright. The mass of downhanging roots from a mat of floating duckweed harbors a microscopic biofilm, which increases the biofiltration capacity and can provide early grazing grounds for the smallest fish fry. J.W. Cross writes, "Except in extreme conditions, low levels of nitrogen, phosphate or trace minerals encourage longer roots, while high fertility results in very short roots, or even an absence of root."
 
Small as it is, Lemna is not the "primitive" plant that it looks like, but has become simplified and miniaturized in the course of its evolution, so that it is no longer even differentiated into stem and leaf tissue. Members of the Lemnaceae are among the world's smallest flowering plants, and a Lemna relative, Wolffia, has the world's smallest true fruit, a bladderlike receptacle smaller than a grain of salt.
 
Duckweed "filtering." Duckweed has the capability of scavenging heavy metals — which is what you call micronutrients when you don't like them — and sequestering them in its tissues. It shades out algae that would cause green water, outcompetes them for nutrients, and  provides refugia for zooplankton that graze on phytoplankton. You can harvest duckweed in a net (or on your forearm hairs) and export the pollutants it represents — to the trash. In this sense, duckweed can be part of your chemical filtration system.
 
Or you can feed it to herbivorous fishes. The protein content of duckweed, dry weight, is in the same range as that of a good flake feed, according to Dr Cross. And the cellulose provides healthy roughage.
 
Some barbs will eat duckweed even faster than it grows. And that's fast: under optimal conditions, duckweed will double in mass every two days. Duckweed will grow in low light or in punishing sun, on a cold outdoor pond and in your steamy Discus tank, at pH down below 5.0 on the one hand and in your Lake Tanganyika aquarium on the other.
 
Links. J.W.Cross's site hosted by the Missouri Botanical Garden, "The Charms of Duckweed", is back after a hiatus, better than ever, with its duckweed information, which includes wastewater management tech using duckweed to reduce nitrates and phosphates and other polluting nutrients. The pix are spectacular, and the biological information is a useful refresher about the world of flowering plants.
 
Lemna is used as a biologist's teaching tool: see Professor Wayne Armstrong's enthusiastic Lemna site.
 
I'm pro-Duckweed myself, but in general aquarists still give Duckweed mixed reviews:  "the occurrence of this Lemna is rarely wanted", Christel Kasselmann writes in Aquarium Plants (2003, p. 332), "due to the mass reproduction under agreeable conditions."  I think it's partly a control issue. But you'd better check out some Duckweed threads at The Krib and less-than-enthusiastic threads on the aquatic-plants mailing list. If your LFS doesn't stoop to carrying duckweed, a generous helping can be ordered from the live food specialists, L.F.S. Cultures. Or a friend can spare a netful.
 
Pistia stratiotes (Water Lettuce). Mine came, through a friend, ultimately from Mexico, though this floating plant, which Linnaeus knew, is worldwide in distribution, in tropical and sub-tropical waters. Outdoors it would swell up into a ribbed cup, but in my tank it formed a minute rosette of leaves with lightly indented outer edges before it slowly faded away. I find Pistia won't co-exist with Water Sprite. Some allelopathic competition seems to be involved, and Water Sprite is the loser. Have you noticed this too?