Anacharis: the classic beginner oxygenator
The bright green stem plant in nearly every starter tank — fast, forgiving, cold-tolerant, and genuinely useful for stripping nutrients out of new water.
Where it comes from
Egeria densa is native to warm, still and slow-moving fresh waters of south-eastern South America — south-eastern Brazil, Uruguay, and northern Argentina. It is a fully submersed stem plant that forms dense stands in ponds, ditches, and the margins of slow rivers.
Those same qualities — rapid growth, tolerance of a huge range of temperatures, and the ability to grow from a single broken fragment — have made it one of the most widely traded aquarium plants in the world under the names anacharis, elodea, and Brazilian waterweed. They have also made it a serious invasive weed well outside its native range. It clogs waterways in the United States, Europe, Australia, and New Zealand, and its sale or possession is restricted or banned in several jurisdictions (it is listed as an invasive species of Union concern in the EU, and is prohibited in a number of US states). Never dispose of trimmings in a natural waterway or storm drain — bag them and put them in general waste.
What to expect
Anacharis produces long, brittle green stems set with whorls of short, strap-shaped leaves — typically four to six leaves per whorl, densely packed toward the growing tip. This whorled arrangement is the quickest way to tell it apart from similar plants: true Elodea canadensis carries its leaves in whorls of three and is a smaller, darker plant, while Lagarosiphon major (sold as "curly" elodea) has leaves that curl backward and are arranged singly, spiralling up the stem.
Plants sold as "anacharis" in the aquarium trade are almost always Egeria densa. There are no ornamental cultivars — what varies is condition. Shop stems grown emersed or in cool water can look sparse and pale; once established under aquarium light they green up and pack their leaves tighter.
How to keep it
| Parameter | Value |
|---|---|
| Lighting | Low to high — grows under almost any light; denser and greener in brighter tanks |
| CO2 | Not required; will grow faster with it but never needs it |
| Temperature | 15–26°C — one of the few tropical-shop plants that also suits unheated coldwater tanks |
| pH | 6.5–8.0 |
| Hardness | Soft to hard (3–20 °dH); can use bicarbonate in hard water |
| Fertiliser | Water-column feeder — responds strongly to nitrate, phosphate and iron dosing |
| Substrate | Any, or none — grows planted or floating |
| Growth rate | Very fast — can add several centimetres a week in good light |
| Placement | Background, Floating |
| Difficulty | Easy |
Why it is sold as an "oxygenator"
Anacharis grows fast enough that its daytime photosynthesis is visible as streams of bubbles (pearling) and it draws heavily on dissolved nutrients. That fast uptake makes it excellent at absorbing ammonia and nitrate in a new or lightly stocked tank, which is the real basis of its "oxygenating" reputation. Remember that at night the same plant consumes oxygen rather than producing it.
In hard water anacharis can perform biogenic decalcification — when free CO2 runs low it strips carbon from bicarbonate, leaving a chalky white crust of calcium carbonate on older leaves. This is harmless and is a sign of vigorous growth in carbonate-rich water, not a deficiency. Adding stable CO2 prevents it if you dislike the look.
Where it works best
Anacharis is a background plant. Planted in loose bunches toward the rear of the tank, the tall stems form a soft green screen and hide equipment. Because the stems are brittle and only root weakly, they are easiest to anchor in a coarse substrate or with a plant weight until roots take hold.
It is equally happy left floating, which is how many keepers use it during tank cycling or in fry and shrimp tanks — floating stems grow fastest, shade the tank, and give baby fish cover. It is not a plant for a precise, manicured aquascape: it grows too fast and too loosely for that. Its place is the low-tech, natural, or coldwater tank where quick coverage matters more than fine control.
How to propagate
Propagation could not be simpler. Cut a healthy stem to length, strip the leaves from the lowest centimetre or two, and replant the top; it will root and grow away. The cut parent stem branches from the node below the cut. Even a fragment that breaks loose will usually root and form a new plant — which is exactly why it is such a successful invader and why trimmings must go in the bin, never a drain or pond.
What goes wrong
Melting or leaf drop after purchase. Stems grown emersed or in cool shop conditions often shed leaves when moved to a warm, brightly lit tank. New growth from the tip is usually healthy — see our guide to plant melt for why this happens and how to ride it out.
Bare, leggy lower stems. Fast top growth shades the base. Trim and replant the tops regularly and remove the leggy lower portions rather than trying to revive them.
Rotting, mushy stems in warm water. Anacharis is a cool-to-temperate plant at heart. Sustained temperatures above about 26–28°C stress it, and stems can soften and rot. If your tropical tank runs hot, a more heat-tolerant fast stem such as hornwort or water wisteria is a better long-term choice.
Pale, thin, stretching growth. Usually too little light or too few nutrients in the water column. Raise light modestly or begin a simple liquid fertiliser routine; anacharis responds quickly to both.
More plants in this series
- Hornwort (Ceratophyllum demersum) — the fast-growing water purifier
- Water sprite (Ceratopteris thalictroides) — lacy fern, plant it or float it
- Amazon frogbit (Limnobium laevigatum) — the easy floating plant
- Water wisteria (Hygrophila difformis) — the shape-shifting stem plant
- Hygrophila polysperma — the ultra-reliable beginner stem