Amazon frogbit: the easy floating plant
Rounded leaves on the surface, long trailing roots below — a fast, forgiving floater that shades the tank, feeds heavily on nitrate, and gives fry and shrimp somewhere to hide.
Where it comes from
Limnobium laevigatum — Amazon or South American frogbit — comes from the still and slow-moving fresh waters of Central and South America, where it floats on the surface of ponds, lakes and quiet river backwaters. Like many floating plants it is a fast coloniser that can form dense surface mats, and it has naturalised as a weed in parts of North America, Europe, Africa and Asia. As with any vigorous floater, never release it into natural waterways; bin unwanted growth.
It is easily confused with true frogbit (Hydrocharis morsus-ranae, a temperate species) and with dwarf water lettuce. Amazon frogbit is distinguished by its glossy, rounded to kidney-shaped leaves and the spongy tissue on their undersides.
Leaves above, roots below
Each plant is a small rosette of rounded leaves 1–3 cm across. Young leaves lie flat on the surface; as they mature they develop a distinctive raised, puffy underside filled with spongy air-storing tissue (aerenchyma) that keeps the plant buoyant. The upper leaf surface is water-repellent and glossy — keeping it dry is central to the plant's health.
Below the surface, frogbit trails fine, feathery roots that can reach 10–30 cm into the water column. These roots are one of the plant's main attractions: they form a shaded, food-rich tangle that shrimp graze on and fry shelter in, and they do much of the plant's nutrient uptake.
How to keep it
| Parameter | Value |
|---|---|
| Lighting | Medium to high — grows fastest and stays compact under bright light |
| CO2 | Not required — it takes CO2 from the air, being a surface floater |
| Temperature | 18–28°C |
| pH | 6.0–7.5 |
| Hardness | Soft to moderately hard |
| Fertiliser | Water-column feeder; benefits from nitrogen, potassium and especially iron |
| Substrate | None — it floats freely |
| Growth rate | Fast — spreads across the surface by runners |
| Placement | Floating |
| Difficulty | Easy |
A nitrate export machine
Because it feeds from the water column and grows fast with unlimited access to atmospheric CO2, frogbit is one of the most effective plants for pulling ammonia and nitrate out of a tank. Scooping out and discarding excess growth every week physically removes those nutrients from the system — a simple, powerful lever for controlling algae in an over-fed tank.
Where it works best
Frogbit is a surface plant, so its "placement" is really about surface management. A patch over one end of the tank creates dappled shade that many fish — especially shy species and labyrinth fish like bettas and gouramis — strongly prefer, while leaving the rest of the surface open for gas exchange and light to the plants below.
The trade-off is exactly that shading: a full frogbit mat blocks a lot of light. If you grow demanding carpets or stems beneath it, thin the frogbit regularly so enough light reaches the substrate. Many keepers corral it behind a floating "fence" (an airline tube loop or a length of suction-cupped rigid tube) to stop it covering the whole surface. It pairs well with other low-fuss species such as anacharis and Java fern below the waterline.
How to propagate
Frogbit propagates itself. Mature plants send out short runners (stolons) that produce daughter rosettes; once a daughter has its own leaves and roots it detaches and floats free. To multiply it deliberately, do nothing — to control it, scoop the excess out. That is the whole of frogbit propagation.
What goes wrong
Melting or rotting leaves. The most common avoidable cause is water sitting on top of the leaves. Condensation dripping from a tight glass lid, spray from a filter outlet or a spray bar, and splashing all wet the water-repellent upper surface and cause it to rot. Leave a gap under the lid for airflow, and direct filter flow so the surface stays calm and dry. New arrivals also often melt for a week or two as they adapt — see plant melt.
Yellowing leaves. Usually a nutrient shortfall — frogbit is hungry, and iron deficiency in particular shows as pale or yellowing leaves. A liquid fertiliser with iron and micronutrients usually greens it up; our guide to chelated iron explains why the form of iron matters.
Small, stunted rosettes. Either too little light or too little nutrient. Frogbit stays largest and healthiest under bright light with adequate feeding.
Taking over the surface. Not a fault so much as its nature — remove a handful weekly. If a dense mat is starving the tank of oxygen or shading plants below, thin it more aggressively.
More plants in this series
- Anacharis (Egeria densa) — the fast, cold-tolerant oxygenator
- Water sprite (Ceratopteris thalictroides) — lacy fern, plant it or float it
- Hornwort (Ceratophyllum demersum) — the fast-growing water purifier
- Java moss (Taxiphyllum barbieri) — the moss that goes anywhere
- Water wisteria (Hygrophila difformis) — the shape-shifting stem plant