Glass Knifefish

Quick summary

The Glass Knifefish is a peaceful, nocturnal species best suited to hobbyists with moderate experience who can provide a 200+ litre tank, dim lighting, plenty of cover, and stable, high-quality water, and who are willing to keep them in groups of five or more rather than alone. It’s not ideal for beginners, since it’s sensitive to poor water quality, may refuse dry foods, and breeding it successfully at home is considered difficult.

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In this guide:

• Introduction
• Quick Stats
• Appearance
• Natural Habitat
• Tank Size and Setup
• Water Parameters
• Filtration and Equipment
• Diet and Feeding
• Behaviour and Temperament
• Tank Mates
• Breeding
• Common Diseases
• FAQs

Introduction

The Glass Knifefish (Eigenmannia virescens) is a fascinating and elegant freshwater species belonging to the order Gymnotiformes, the New World electric knifefish. It is important to clarify from the outset that this species is entirely unrelated to the Old World featherback knifefish of the family Notopteridae, despite sharing the common name “knifefish.” The Gymnotiformes are a distinctly South American lineage of weakly electric fishes, and the Glass Knifefish is one of the most widely available and rewarding members of this group for the home aquarium.

This species is also commonly sold under the name Green Knifefish. These are not different colour morphs or separate species – they are the same fish, Eigenmannia virescens. The name “Glass” references the fish’s remarkably transparent, see-through body, through which internal organs and the spine are clearly visible. The name “Green” derives from a faint natural greenish tint that some individuals display, particularly under certain lighting conditions. Both names are used interchangeably in the aquarium trade, and hobbyists should be aware that purchasing either will yield the same species.

Native to a vast range across South America, the Glass Knifefish is a social, nocturnal species that uses weak electrical signals to navigate its environment and communicate with conspecifics. Lacking a dorsal fin entirely, it propels itself with a mesmerising undulating motion of its exceptionally long anal fin, allowing it to glide forwards and backwards with equal grace. This unusual mode of locomotion, combined with its ethereal translucent appearance, makes it one of the more captivating species available to freshwater fishkeepers. Though not a beginner’s fish in the strictest sense, the Glass Knifefish is well within the reach of hobbyists with moderate experience, provided its requirements for group housing, subdued lighting, and stable water quality are met.

Appearance

The Glass Knifefish possesses one of the most distinctive body plans in the freshwater aquarium hobby. Its body is extremely laterally compressed and elongated, tapering to a fine point at the tail. There is no caudal fin in the traditional sense, and the dorsal fin is entirely absent. Instead, the fish is propelled almost exclusively by its extraordinarily long anal fin, which runs from just behind the pectoral fins all the way to the tip of the tail, spanning the vast majority of the body’s underside. This fin ripples in continuous, elegant undulations, enabling the fish to move forward, backward, and hover in place with remarkable precision.

The most striking feature of this species is its transparency. The body is largely see-through, with the internal organs visible as a silvery mass concentrated in the anterior portion of the body near the head. The vertebral column is clearly visible as a fine line running along the length of the fish. The overall body colour is a pale, glassy silver with a faint greenish or yellowish iridescence that can become more pronounced depending on lighting, diet, and the individual’s condition. This subtle green sheen is the source of the alternative common name “Green Knifefish.”

Genuine sexual dimorphism exists in this species and is useful for sexing individuals. Males tend to develop a warmer, yellowish tone to their body, particularly as they mature and approach breeding condition. Females, by contrast, typically exhibit a cooler grey-white colouration. Males also tend to grow slightly larger than females and may develop a somewhat deeper body. These differences become more pronounced in mature, well-conditioned fish and can be a reliable means of distinguishing the sexes, especially when the fish are kept in groups where comparisons can be made directly.

The head is small relative to the body, with a slightly upturned mouth and moderately sized eyes. The pectoral fins are relatively small and transparent. When healthy and well-kept, the Glass Knifefish has a luminous, almost ghostly quality, particularly when viewed under subdued or moonlight-style lighting that highlights the translucent body and the gentle shimmer of its iridescence.

Natural Habitat

The Glass Knifefish is one of the most geographically widespread species within the Gymnotiformes, found across an enormous range in South America. Its distribution encompasses the Amazon basin, the Orinoco basin, the Parana and La Plata river systems, and numerous smaller drainages in between. It has been recorded in countries including Brazil, Peru, Colombia, Venezuela, Bolivia, Paraguay, Argentina, Uruguay, and the island of Trinidad. This broad distribution reflects the species’ considerable adaptability, although its core habitat preferences remain fairly consistent throughout its range.

In the wild, Glass Knifefish are typically found in slow-moving or still waters, including flooded forests, backwater pools, marshes, oxbow lakes, and the vegetated margins of rivers and streams. They strongly favour areas with dense aquatic vegetation, submerged root systems, leaf litter, and overhanging bankside plants. These habitats provide the dim, sheltered conditions that suit their nocturnal lifestyle. During daylight hours, groups of Glass Knifefish rest together in dense cover, becoming active at dusk and throughout the night.

The waters they inhabit are typically warm, soft to moderately hard, and range from slightly acidic to neutral in pH. Many of the habitats are tannin-stained blackwater or clearwater environments with substrates of sand, mud, or decomposing leaf litter. The dense vegetation and shaded conditions mean that light levels in their natural environment are characteristically low, a factor that should inform the aquarium setup.

As weakly electric fish, Glass Knifefish generate a continuous low-voltage electric field using specialised cells called electrocytes in their tail region. This electric organ discharge (EOD) serves dual purposes: electrolocation, which allows the fish to navigate and detect objects and prey in dark, turbid water, and electrocommunication, through which individuals signal to one another. Each fish produces a wave-type EOD at a specific frequency, and these signals play an important role in social interactions, territory establishment, and mate selection. In their natural habitat, Glass Knifefish are highly social and are almost always found in groups, often numbering in the dozens or even hundreds.

Tank Size and Setup

A minimum tank size of 200 litres is recommended for a small group of Glass Knifefish. Given that this is a social species that should ideally be kept in groups of five or more, larger tanks of 300 litres or above are strongly preferred. A longer tank footprint is more valuable than height, as the fish spend the majority of their time in the lower and middle water columns and benefit from horizontal swimming space. A tank measuring 120 cm or more in length is ideal.

The aquascape should prioritise providing abundant cover and subdued lighting. Dense plantings of tall-growing species such as Vallisneria, Echinodorus, and Cryptocoryne work well, as do floating plants like Amazon frogbit or water lettuce, which help to diffuse overhead light. Driftwood, bogwood, and tangles of roots or branches should be arranged to create sheltered resting spots and visual barriers. PVC tubes, clay pipes, or coconut shells can also serve as daytime hiding places and are readily accepted by this species, though a more naturalistic approach with wood and plants is generally more aesthetically pleasing.

The substrate should be fine sand or smooth gravel. Glass Knifefish sometimes rest on the bottom and have delicate, elongated bodies that can be irritated by sharp or rough substrates. A sandy substrate also supports the growth of beneficial microorganisms and allows for more natural foraging behaviour.

Subdued lighting is essential. As a nocturnal species, the Glass Knifefish is naturally averse to bright light and will remain hidden and stressed if the tank is too brightly illuminated. Floating plants, dimmer LED settings, or a canopy of tall plants should be used to create shaded zones. Moonlight or blue-spectrum LED lighting can be an excellent addition, as it allows the keeper to observe the fish during their most active period without causing them distress.

A secure, well-fitting lid is important. While Glass Knifefish are not notorious jumpers in the way that some other species are, they can and do leap from the water if startled, and the risk increases in an open-top aquarium, particularly during the first few weeks after introduction when the fish are still acclimatising.

Water Parameters

The Glass Knifefish is reasonably adaptable in terms of water chemistry, reflecting its broad natural distribution across many different water types. However, it does best in soft to moderately hard, slightly acidic to neutral water. A temperature of 24 – 26 °C is ideal for general maintenance, with slightly warmer temperatures useful during breeding attempts.

Water quality must be kept consistently high. Glass Knifefish are sensitive to elevated levels of dissolved organic waste, and prolonged exposure to ammonia, nitrite, or high nitrate levels will compromise their health and may prove fatal. Regular partial water changes of 25 – 30% weekly are recommended. The use of high-quality dechlorinated water is essential, and hobbyists using tap water should test regularly for chloramine as well as chlorine, as the former is not always removed by standard dechlorinators.

The addition of tannins through the use of Indian almond leaves, dried oak leaves, or alder cones can be beneficial. These replicate the blackwater conditions found in many of the species’ natural habitats, lower pH gently, and have mild antibacterial and antifungal properties. The resulting tea-coloured water also helps to reduce light levels in the tank, which is beneficial for this light-shy species.

Filtration and Equipment

Efficient biological filtration is the single most important piece of equipment for a Glass Knifefish aquarium. A canister filter rated for the tank’s volume – or ideally oversized by 50% or more – is the best choice, as it provides excellent biological media capacity and can be configured to produce a gentle, diffused output. Glass Knifefish are not strong swimmers in the conventional sense and do not appreciate powerful currents. The filter output should be directed against the glass, spread with a spray bar, or otherwise diffused to create a gentle flow throughout the tank.

An additional sponge filter can be a useful supplement, providing extra biological filtration surface area and gentle aeration without adding to the current. Sponge filters are also beneficial in breeding setups, as they are safe for fry and do not create suction strong enough to trap small fish.

A reliable heater with a thermostat is necessary to maintain stable temperatures. Sudden temperature fluctuations are stressful for this species, and a heater guard is advisable to prevent the fish from resting against the heater element and sustaining burns, a risk that is heightened with scaleless or near-scaleless fish like knifefish.

As discussed above, lighting should be subdued. Programmable LED units with dimming and moonlight settings are ideal, as they allow the keeper to simulate a natural dusk-to-dawn cycle and observe the fish during their active nocturnal period. A timer is essential to ensure a consistent photoperiod, typically 8 – 10 hours of light per day with a gradual dimming transition rather than an abrupt switch-off.

It is worth noting that because Glass Knifefish are weakly electric, there is a common concern among hobbyists about whether electrical equipment in the tank can interfere with or be affected by the fish’s electric organ discharges. In practice, the voltages generated by Eigenmannia virescens are far too low (typically less than one millivolt) to have any measurable effect on aquarium equipment, and standard heaters, filters, and lighting will not interfere with the fish’s electrolocation abilities. There is no need for any special equipment considerations on this account.

Diet and Feeding

In the wild, Glass Knifefish are primarily insectivorous and microphagous, feeding on small aquatic invertebrates, insect larvae, worms, and tiny crustaceans that they locate using their electrosensory system in the dark. In the aquarium, they retain strong preferences for live and frozen foods and can be reluctant to accept dry prepared diets, at least initially.

The staple diet should consist of high-quality frozen foods such as bloodworm, daphnia, brine shrimp, mosquito larvae, and tubifex. Live foods are eagerly accepted and are an excellent way to encourage natural foraging behaviour and bring the fish into breeding condition. Live blackworms, grindal worms, white worms, and baby brine shrimp are all suitable. Chopped earthworms can also be offered to larger individuals.

Some hobbyists report success in gradually weaning Glass Knifefish onto high-quality sinking pellets or granules, particularly those formulated for carnivorous or insectivorous species. This is more likely to succeed if the dry food is introduced alongside familiar live or frozen items and offered during the dim evening period when the fish are most active and willing to feed. However, not all individuals will accept prepared foods, and a keeper should be prepared to maintain a diet based primarily on frozen and live offerings.

Feeding should take place in the evening or after lights-out. As a nocturnal species, the Glass Knifefish is most active and willing to feed during dim or dark conditions. Feeding during the bright-light period will often result in the food going uneaten or being consumed only by diurnal tank mates, with the knifefish missing out. Using moonlight-spectrum lighting during feeding allows the keeper to observe the fish eating while respecting their natural activity cycle.

Feed once or twice daily in quantities that the fish can consume within a few minutes. Overfeeding should be avoided, as uneaten food will decompose and degrade water quality, to which this species is sensitive. In a community tank, take care to ensure that food reaches the knifefish and is not monopolised by faster, more assertive diurnal species.

Behaviour and Temperament

The Glass Knifefish is a peaceful, social, and predominantly nocturnal species. During the day, it typically rests in cover – among plant stems, beneath driftwood, inside tubes, or wedged into narrow crevices. Groups will often rest together in close proximity, sometimes with their bodies touching or overlapping. This gregarious daytime behaviour is one of the more endearing aspects of the species and is best appreciated when the fish are kept in groups of five or more.

At dusk, the fish become increasingly active, emerging from cover to explore the tank and search for food. Their swimming style is extraordinarily graceful, with the body held rigid and near-horizontal while the long anal fin produces a continuous sinusoidal wave that propels the fish forward, backward, or allows it to hover motionless in the water column. This “knife” swimming style is characteristic of all Gymnotiformes and is a source of considerable fascination for observers.

As weakly electric fish, Glass Knifefish are constantly generating and receiving electrical signals. Within a group, individuals will modulate their EOD frequencies to avoid interference with one another, a phenomenon known as the “jamming avoidance response.” This is one of the most extensively studied behaviours in neuroethology and is a remarkable example of real-time sensory processing and social signalling. While the keeper cannot perceive these signals without specialised equipment, the behaviours they drive – including subtle body postures, approach-withdrawal interactions, and changes in activity level – are observable and add depth to the fishkeeping experience.

Aggression within the species is generally limited. Males may establish a loose dominance hierarchy, particularly in smaller groups, and some chasing or mild sparring can occur, but serious injuries are rare. Keeping a group of five or more individuals helps to diffuse any tension and allows for more natural social dynamics. A solitary Glass Knifefish will often be shy, stressed, and reluctant to feed, and is far more likely to remain permanently hidden.

Towards other species, the Glass Knifefish is almost entirely peaceful. It will not bother fish that are too large to be considered prey, and its small mouth limits predation risk to very tiny fry or miniature invertebrates. It is not territorial in the conventional sense, although it may compete with other nocturnal, bottom-dwelling species for preferred resting spots.

Tank Mates

The Glass Knifefish’s peaceful, nocturnal nature makes it compatible with a wide range of community fish, provided certain considerations are met. Tank mates should be non-aggressive, not overly boisterous, and ideally not so fast and competitive at feeding time that the knifefish are unable to secure food. Very small fish or fry may be at risk of predation, particularly during the nocturnal period, though the Glass Knifefish’s relatively small mouth limits this to truly tiny species. Avoid keeping them with large, aggressive, or highly territorial cichlids, or with other strongly electric species that may interfere with their electrosensory system.

Good tank mates

• Cardinal Tetra – A peaceful, similarly soft-water-loving species that occupies the mid-water column and does not compete with knifefish for space or food.
• Rummy-Nose Tetra – A gentle schooling fish that thrives in the same warm, soft water conditions and adds visual interest during the daytime.
• Corydoras sterbai – A peaceful, warm-water-tolerant bottom-dweller that coexists well with knifefish and helps to keep the substrate clean.
• Bristlenose Pleco – A hardy, non-aggressive algae eater that stays at a manageable size and does not disturb knifefish.
• Harlequin Rasbora – A calm, shoaling species well suited to the soft, slightly acidic water preferred by Glass Knifefish.
• Pearl Gourami – A peaceful labyrinth fish that does well in planted, dimly lit aquaria and is not aggressive towards other species.
• Kuhli Loach – Another nocturnal, peaceful species that occupies the bottom of the tank and coexists well in densely planted setups.
• Bolivian Ram – A mild-mannered dwarf cichlid that tolerates similar water conditions and is not aggressive enough to bother knifefish.

Fish to avoid

• Oscar – A large, aggressive predator that would view a Glass Knifefish as potential prey and create a highly stressful environment.
• Jack Dempsey – An aggressive, territorial cichlid that will harass and potentially injure the peaceful, slow-moving knifefish.
• Tiger Barb – A notorious fin-nipper whose boisterous behaviour can stress and damage the knifefish’s delicate anal fin.
• Black Ghost Knifefish – While superficially a logical companion, this larger, more territorial knifefish species can be aggressive towards other electric fish and may cause electrosensory interference.
• Red Devil Cichlid – A highly aggressive, large cichlid wholly incompatible with any peaceful community species.
• African Clawed Frog – An opportunistic predator that may attempt to consume knifefish during the nocturnal period when both are active.

Breeding

Breeding the Glass Knifefish in captivity is achievable but challenging, and successful spawnings remain relatively uncommon in the hobby. The species is an egg scatterer, and reproduction in the wild is triggered by seasonal flooding and the associated changes in water chemistry, temperature, and food availability.

The first step towards breeding is to establish a group of healthy, mature adults and to correctly identify males and females. As noted in the Appearance section, males tend to develop a yellowish body tone, while females are typically grey-white in colouration. Males also tend to be slightly larger and may have a marginally deeper body profile. These differences become more apparent when the fish are in breeding condition, and keeping them in a group makes side-by-side comparison straightforward.

To condition the fish for spawning, increase the proportion of live foods in the diet over a period of several weeks. Live blackworms, earthworm pieces, and mosquito larvae are excellent conditioning foods. Simultaneously, begin to gradually lower the water level in the tank and slightly raise the temperature to 27 – 28 °C. After a conditioning period, a large water change (50% or more) with soft, slightly cooler water can be used to simulate the onset of the rainy season and trigger spawning behaviour. Some breeders also darken the tank further and increase aeration during this period.

Spawning typically occurs during the night or in very dim light. Males court females with changes in their electrical discharge frequency and with physical displays, swimming alongside the female and making close body contact. Eggs are scattered among fine-leaved plants or over the substrate. The eggs are small, semi-adhesive, and translucent. Neither parent provides any care for the eggs or fry, and there is a risk of the adults consuming the eggs if they are left in the same tank.

If eggs are observed, they should ideally be removed to a separate rearing tank with gentle sponge filtration, matching water parameters, and very dim lighting. The eggs typically hatch within 48 to 72 hours, depending on temperature. The fry are tiny and require infusoria or commercially available liquid fry food for the first few days, graduating to freshly hatched baby brine shrimp and microworms as they grow. Growth is relatively slow, and the fry are delicate, requiring stable water conditions and frequent small feeds.

Common Diseases

The Glass Knifefish is not especially disease-prone when kept in appropriate conditions, but it is sensitive to poor water quality and is somewhat more vulnerable to certain ailments than hardier community species. Its lack of prominent scales and its transparent, delicate skin make it particularly susceptible to skin infections and parasitic infestations if water conditions deteriorate.

Ich (white spot disease) is one of the most common afflictions, particularly following the stress of transport, introduction to a new tank, or sudden temperature drops. The characteristic white cysts are easily visible on the knifefish’s translucent body. Treatment should be approached with caution, as Glass Knifefish can be sensitive to copper-based and harsh chemical medications. Heat treatment (raising the temperature to 30 °C over 24 – 48 hours) combined with aquarium salt at low concentrations is often preferred, though the use of salt should be conservative as the species is somewhat salt-sensitive. Half-dose treatments of malachite green-based medications are an alternative.

Bacterial infections, including fin rot and skin ulcerations, can occur in fish kept in suboptimal water quality or in tanks with sharp decor that may cause abrasions. Maintaining pristine water conditions and using smooth, rounded decor and a fine sand substrate are the best preventative measures. Mild bacterial infections can often be resolved by improving water quality alone, while more serious cases may require treatment with antibacterial medications formulated for sensitive species.

Internal parasites can occasionally be a problem, particularly in wild-caught specimens. Symptoms may include weight loss despite a healthy appetite, white or stringy faeces, and a general decline in condition. Antiparasitic medications such as praziquantel or levamisole can be effective, but dosing should be carefully managed for this sensitive species. Quarantining all new arrivals for a minimum of two to four weeks is strongly recommended.

Stress-related issues are arguably the most common health concern in Glass Knifefish. Fish that are kept singly, in brightly lit tanks, or without adequate hiding places will often display chronic stress, manifested as persistent hiding, refusal to feed, pale or washed-out colouration, and heightened susceptibility to opportunistic infections. Addressing the root causes of stress – providing a group, dim lighting, and abundant cover – is invariably more effective than medicating the symptoms.

FAQs

Are Glass Knifefish and Green Knifefish the same species?

Yes, they are the same species, Eigenmannia virescens. The name “Glass” refers to the fish’s transparent body, while “Green” comes from a subtle natural greenish tint that some individuals display. These are not different colour morphs or separate species – both names describe the same fish and are used interchangeably in the aquarium trade.

Do Glass Knifefish need to be kept in groups?

Yes. The Glass Knifefish is a naturally social species that lives in groups in the wild. Keeping them singly or in pairs often results in chronic stress, persistent hiding, and refusal to feed. A group of five or more is recommended. In a group, they will be more confident, more active, and will display their fascinating social behaviours, including their electrical communication. A larger group also helps to diffuse any minor hierarchical aggression among males.

Can Glass Knifefish shock you or other fish?

No. Glass Knifefish are weakly electric, generating extremely low-voltage discharges (typically less than one millivolt) that are used solely for navigation and communication. These signals are far too weak to be felt by humans or to cause any harm to other fish. They are entirely different from the powerful discharges produced by strongly electric species such as the Electric Eel. You will not receive a shock from handling or netting a Glass Knifefish.

Why does my Glass Knifefish hide all day?

This is entirely normal behaviour. The Glass Knifefish is a nocturnal species that rests in cover during daylight hours and becomes active at dusk and throughout the night. If you wish to observe your fish more frequently, consider installing moonlight or blue-spectrum LED lighting that can be used during the evening, as this allows you to watch the fish during their natural active period without causing them stress. Ensuring adequate cover, dim lighting, and group housing will also increase the fish’s overall confidence, and they may begin to emerge earlier in the evening.

Will Glass Knifefish eat shrimp?

Glass Knifefish are likely to prey on small ornamental shrimp such as Cherry Shrimp or Crystal Red Shrimp, particularly during the nocturnal period when the knifefish are actively hunting. While their mouths are relatively small, they are natural predators of small invertebrates and will readily consume shrimp that they can catch and fit into their mouths. Larger shrimp species such as Amano Shrimp may fare somewhat better, but the risk remains. If you wish to keep shrimp, it is best to house them separately from Glass Knifefish.

Related Guides

• Black Ghost Knifefish – Another popular Gymnotiform knifefish with similar care requirements but a larger adult size and more solitary temperament.
• Cardinal Tetra – An excellent companion species for the Glass Knifefish, sharing preferences for soft, acidic water and planted tanks.
• Kuhli Loach – A fellow nocturnal, peaceful bottom-dweller that thrives in the same type of dimly lit, densely planted aquarium.
• Corydoras sterbai – A warm-water-tolerant catfish that makes an ideal bottom-dwelling companion in a Glass Knifefish community tank.
• Bristlenose Pleco – A manageable-sized pleco suitable for the planted, dimly lit setups that Glass Knifefish prefer.