Salvinia Care Guide (Salvinia sp.)
Salvinia is a group of small free-floating ferns often added to a freshwater tank for shade, nutrient control and surface cover. A floating plant is simply one that is not rooted in the substrate at all but drifts at the surface, drawing its food straight from the water. Each plant carries its paired oval leaves – clothed in a velvety coat of tiny, water-repellent hairs – flat on the surface, while a fringe of fine, feathery submerged fronds hangs beneath like roots. Hobbyists like salvinia because it grows quickly and easily, mops up the excess nutrients that would otherwise feed algae, and shelters fry and shrimp among its trailing fronds. There is, however, one very important catch with the plant sold in the trade as Giant Salvinia, and it is a legal one that every Australian keeper needs to understand before buying. We cover it plainly below.
1. Species Overview
Salvinia belongs to the fern family Salviniaceae, and there are several species in and around the aquarium trade. The smaller, commonly kept ones include Salvinia natans (the temperate common floating fern, native to Europe and Asia), Salvinia minima (commonly sold as "water spangles") and the closely related Salvinia auriculata, both native to tropical America, and Salvinia cucullata from Asia. All share the same layout: paired floating leaves with a distinctive coat of water-repellent hairs on top, and a skirt of dangling, root-like submerged fronds. Those surface hairs are the main way botanists tell the species apart – on giant salvinia the four tips of each hair rejoin at the top into a cage- or egg-beater shape, whereas on the smaller species the tips stay separate. This plant is sold in the trade as Giant Salvinia (Salvinia molesta), also known as Kariba weed or aquarium watermoss, a species native to south-eastern Brazil that sits within the S. auriculata complex – which is exactly why the two are so easily confused; it is not a species we stock, for the legal reasons set out below. Giant salvinia is sterile and never sets viable spores; it spreads entirely by fragmentation, every broken piece able to grow into a new plant. That single trait, together with a growth rate that lets it double in days and pile into thick, layered mats, is what makes it one of the world's worst aquatic weeds and, in Australia, a declared plant.
2. Difficulty Level and Suitability
As a plant to grow, salvinia is easy, and the care sources are unanimous: Tropica and Flowgrow both rate it easy, it needs no carbon dioxide injection, no substrate and no special skill, and because it floats at the top of the tank it is never shaded out by taller plants. If anything the only real challenge is keeping it in check. In Australia, though, suitability is not really a growing question at all – it is a legal one, and it is the single most important thing on this page. Giant salvinia (Salvinia molesta) is one of Australia's Weeds of National Significance, and according to Weeds Australia (weeds.org.au) it is declared in every Australian state and territory. In practice that means, for example, that in Victoria it is a State prohibited weed – Agriculture Victoria (agriculture.vic.gov.au) states plainly that it is illegal to buy, sell, display or move the plant. In Queensland it is a category 3 restricted invasive plant under the Biosecurity Act 2014; Business Queensland states you must not give it away, sell it or release it, and that everyone has a general biosecurity obligation to limit its spread – and Queensland treats every other Salvinia species as prohibited matter that must be reported. In New South Wales, NSW WeedWise (the state's Department of Primary Industries) states it must not be imported into the state, sold, bartered, exchanged or offered for sale, and a general biosecurity duty applies right across the state. We are not lawyers and this is not legal advice, but the plain factual position is that giant salvinia cannot lawfully be sold or moved in much of the country. Please check the current rules for your own state or territory before acquiring or keeping any Salvinia. If you simply want a floating fern for its looks and its knack for nutrient control, the smaller species (S. natans, S. minima, S. auriculata) are the usual, far less aggressive aquarium plants internationally – but do not assume they are automatically legal here either, because some jurisdictions, Queensland among them, regulate the whole genus. When in doubt, ask our team or your state agriculture or biosecurity department.
3. Water Parameters
Steady, clean water matters more to salvinia than hitting any exact figure, and the plant is genuinely tolerant, so treat the numbers below as comfortable targets rather than strict limits. The ranges published by different sources vary, which is itself a sign of how adaptable a floating fern is.
| Parameter | Target range | Notes |
|---|---|---|
| Temperature | 18–28 °C | Undemanding. aquariumbreeder lists 22–26 °C, while aqualandpetsplus and Canton Aquatics give 18–28 °C. The tropical species (S. auriculata, S. minima) prefer the low-to-mid 20s; S. natans is more temperate and takes cooler water. Weeds Australia notes giant salvinia grows fastest at 20–30 °C and slows below about 10 °C. |
| pH | 6.0–7.5 | Adaptable. aqualandpetsplus and Canton Aquatics give 6.0–7.5 with an optimum near neutral, while aquariumbreeder records tolerance up to about 8.0. Slightly acidic to neutral suits it best, but most tap water is fine. |
| GH (general hardness) | 3–15 dGH | GH is the calcium and magnesium in the water. Not fussy: aquariumbreeder lists roughly 3–18 dGH and aqualandpetsplus 4–15 dGH. Soft to moderately hard water is ideal, and harder water rarely troubles it. |
| KH (carbonate hardness) | ≈3–8 dKH | KH is the water's buffering capacity. Published figures are limited – aqualandpetsplus gives about 3–8 dKH – and as a surface floater it is undemanding here; a stable KH matters more than the exact number. |
| Nutrients / substrate | Water-column feeder | Being free-floating, salvinia takes everything through its submerged fronds directly from the water, so a nutrient substrate or root tabs do nothing for it. It is a hungry plant that strips nitrogen, phosphate and iron – which is exactly what makes it such a good nutrient-control tool. |
The one thing to watch is that, as a fast and heavy feeder, salvinia needs some nutrients actually present in the water. In a very lean, low-nitrate tank the newer leaves can pale and shrink; keeping a little nitrate and a comprehensive trace mix, including iron, in the water keeps the foliage a healthy green.
4. Lighting Requirements
On the same PAR framework our coral guides use (PAR is photosynthetically active radiation, a measure of usable light reaching the plant), low light for plants is roughly 30–50 PAR, medium 50–80 and high 80 or more. Salvinia sits right beneath the light at the surface, so it effectively receives strong light no matter how the fixture is rated – which is why Tropica lists its light demand as low (around 0.5 watts per litre) and even a modest 30–50 PAR at the surface is ample. Other sources call for moderate to high light for the densest growth, and that spread barely matters in practice for a plant floating at the very top of the tank. More light produces a thicker, more compact mat and can bring out a bronze or reddish tint in some forms, while low light gives thinner, paler growth. Because the leaves sit at the surface rather than acclimatising from below, there is no need to ease it into bright light the way you would a submersed plant – though a brand-new, very intense fixture can still scorch floating leaves, so bring strong lighting up gradually if you are in doubt. The catch runs the other way, too: a thick raft of salvinia is itself a very effective shade and will cut the light reaching plants growing below it, so thin it to control how much passes through.
5. CO₂ and Fertilisation
Plants grow by photosynthesis, using light to combine carbon dioxide and dissolved nutrients into new tissue, so more available carbon usually means faster growth. Salvinia, though, has an advantage that rooted submersed plants lack: because its leaves sit on the surface with their tops in the air, it can draw carbon dioxide straight from the atmosphere. This is a large part of why floating plants grow so vigorously without any help, and it means CO₂ injection is not required – Tropica lists its carbon requirement as low (0–5 mg/L) and most keepers run it with none at all. What salvinia does want is feeding. It is a heavy water-column feeder, so a comprehensive all-round liquid fertiliser carrying nitrogen, phosphorus, potassium, iron and trace elements keeps it lush; iron in particular keeps the leaves a fresh green rather than pale. Root tabs and nutrient substrates are wasted on a plant with no substrate roots. Its healthy appetite for nitrate and phosphate is exactly why it works so well as a nutrient sponge and an algae-suppressant, quietly out-competing algae for the same food.
6. Planting and Placement
There is nothing to plant – salvinia is used exactly one way, floating free on the surface, so you simply rest it on the water and let it drift; never bury it or weigh it down. Give it an open patch of calm surface, because if it packs tightly against the glass or the filter outlet the crowded, wetted leaves tend to rot. Left to itself it will spread until it carpets the whole surface, cutting the light to the plants below and slowing gas exchange at the top, so many keepers pen it into one area with a floating ring, a loop of airline tubing or a purpose-made surface divider, which keeps an open patch for viewing and for oxygen exchange. Plan to thin it every week or two rather than letting it seal the tank over. Because it casts such useful shade, salvinia pairs well with low-light plants such as anubias, ferns and crypts underneath, and with fish that appreciate dimmer, calmer water; the trailing fronds also make excellent cover for fry and shy surface-dwellers.
7. Growth Rate and Propagation
Growth is fast – Tropica rates it high and Flowgrow calls the auriculata group very fast – and giant salvinia faster still, which is precisely the problem it poses in the wild. Propagation needs no effort on your part, because the plant does all the work itself: each one branches and breaks into new plants as it grows, so a small starter portion becomes a thick mat within weeks. To multiply it deliberately you need only split a clump and move half, or simply scoop off the excess as part of your routine maintenance. That ease of spread is the flip side of its weed risk – every fragment is a potential new colony – which is why the plant needs regular thinning far more than it needs any encouragement to grow, and why disposing of the excess responsibly is not optional. The next section covers how.
8. Common Issues and Troubleshooting
The most common complaint is rotting or melting leaves – leaves that go brown, translucent and mushy, then break apart. Salvinia's leaf tops are water-repellent and are meant to stay dry; constant droplets from condensation, spray or a splashing filter return will waterlog and rot them. Angle your outflow so it does not spray the surface, keep surface agitation gentle, wipe or leave a gap under the cover glass so condensation does not rain back down, and do not let the mat jam against the filter outlet. Pale, yellowing or shrinking leaves, especially on the newest growth, point to a nutrient shortage rather than a disease – dose a complete liquid fertiliser with iron and make sure some nitrate is present. A little melt shortly after you introduce the plant is normal as it settles from transport. The problem that grows with success is overgrowth: a complete surface blanket starves the plants below of light and can reduce oxygen exchange at the surface, so thin it regularly and never let it seal the tank over entirely. Finally, and most importantly, dispose of salvinia responsibly and never release it. Do not tip it, or any water that may contain fragments, into a creek, river, drain, stormwater system or any natural waterway – a single fragment can start a new infestation, and dumped aquarium and pond plants are a major reason giant salvinia has spread as far as it has. To get rid of excess, remove it, let it dry out completely, then seal it in a bag for the general rubbish or compost it well away from any water. Given that giant salvinia is a Weed of National Significance declared across Australia, if you believe you have it you should check your state or territory's biosecurity or agriculture guidance on what you are required to do, because in some places you may be obliged to report it. When you are unsure, contact our team or your local biosecurity authority rather than guessing.
A Note on the Ranges Above
The parameters and other figures in this guide are a general guideline only. Every planted aquarium is different, and the ideal balance for salvinia depends on your lighting, your substrate, your other plants and whether you run CO₂. For help working out tank volume and the right dose of fertiliser, plant food and treatments for your system, have a look at our Aquarium Calculators.
Disclaimer
This guide is provided for general educational purposes only. It is not a substitute for professional advice, and we cannot guarantee outcomes. Individual plants vary, and the right conditions for your system depend on your lighting, your equipment and the other livestock and plants you keep. If you are unsure about anything, please get in touch with our team before proceeding.