Eigenmannia virescens 5 - (Eigenmannia virescens) — 5-8cm

Overview

Knife fish enthusiasts seeking a graceful, nocturnal species will find Eigenmannia virescens, commonly known as the glass knifefish or green knifefish, a captivating choice for specialist aquariums. This remarkable South American species originates from the Amazon and Orinoco basins, where it inhabits slow-moving waters with dense vegetation and abundant cover. Available for sale UK in sizes from 5-12cm, the species reaches approximately 35cm at maturity, requiring substantial space and long-term planning. The semi-transparent body, unique undulating fin movement, and fascinating electric sense distinguish this species from conventional aquarium fish. Moderate care requirements combined with the need for dim lighting, calm tankmates, and patient acclimation suit experienced aquarists prepared to provide a species-appropriate environment.

Appearance

Eigenmannia virescens displays a unique elongated body that appears laterally compressed and semi-transparent, revealing internal structures under certain lighting. Body coloration ranges from pale olive-green to yellow-grey with a translucent quality that gives rise to the "glass" common name. The most distinctive feature is the extremely long anal fin that runs nearly the entire length of the body from throat to tail, creating continuous undulating waves that propel the fish forward and backward with equal ease. The dorsal fin is absent, and the caudal fin is reduced to a small filament. The head is small and pointed, with large eyes positioned slightly forward. Barbels or whisker-like projections are absent. The species moves with liquid grace, hovering motionless or gliding smoothly in any direction. At night, the fish becomes more active and alert, reflecting its crepuscular and nocturnal lifestyle.

Natural Habitat

This species inhabits slow-moving rivers, floodplain lakes, tributaries, and densely vegetated backwaters throughout the Amazon and Orinoco basins in South America. Natural habitats feature submerged wood, root systems, aquatic vegetation, and overhanging riparian plants that create shaded, protected environments. Water is typically warm, soft, slightly acidic, and often tannin-stained. The species is primarily nocturnal, sheltering among vegetation or submerged structures during daylight and emerging at dusk to hunt. Eigenmannia virescens is a weakly electric fish, generating and detecting electric fields to navigate, locate prey, and communicate with conspecifics in low-visibility conditions. This electrosensory ability allows the species to thrive in murky, dark waters where vision is limited.

Aquarium Setup

A minimum 300-litre aquarium with dimensions emphasizing length provides appropriate space for a single adult glass knifefish or a small compatible group. Use fine sand or smooth substrate and provide abundant hiding places using driftwood, PVC tubes, ceramic caves, and dense planting with species such as Java fern, Anubias, and Vallisneria. Subdued, dim lighting is essential—bright light stresses the fish and inhibits natural behavior. Floating plants or strategically positioned wood help diffuse light and create shadowed areas. Gentle filtration maintains water quality without creating strong currents, as the species prefers calm water. Ensure the tank has a secure lid, as knifefish may jump when startled. Maintain excellent water quality, as the species is sensitive to ammonia, nitrite, and elevated nitrates.

Water Parameters

Maintain temperatures between 24-28°C, with 25-27°C representing optimal conditions. The species tolerates slight variations but requires stability. pH should remain between 6.0-7.5, with slightly acidic to neutral values (6.5-7.0) ideal. Water hardness can range from soft to moderately hard, though soft to moderate hardness better replicates natural Amazonian conditions. Ammonia and nitrite must be undetectable, while nitrates should stay below 30 ppm through efficient biological filtration and weekly water changes of 30-40%. The species is sensitive to poor water quality and deteriorating conditions, responding with lethargy, loss of appetite, or increased hiding. Maintain consistency and perform regular maintenance to ensure long-term health.

Feeding

Eigenmannia virescens is a carnivore requiring a diet of small live or frozen meaty foods. Offer bloodworms, blackworms, brine shrimp, daphnia, mosquito larvae, and other appropriately sized invertebrate foods. Live foods are particularly effective for encouraging feeding, especially during initial acclimation. Feed during evening or nighttime hours to align with the species' nocturnal activity pattern. Some individuals may eventually accept high-quality sinking carnivore pellets after a gradual transition, but live or frozen foods should form the dietary foundation. Feed small portions 4-5 times weekly rather than daily, as the fish has a slow metabolism. Ensure food reaches the bottom or midwater zones where the knifefish feeds, as it rarely surface feeds. Monitor body condition—a healthy specimen appears streamlined but not emaciated.

Behaviour

Glass knifefish are primarily nocturnal and crepuscular, remaining hidden during daylight hours and emerging during evening, nighttime, and early morning periods. The species moves with extraordinary grace, hovering motionless or gliding smoothly forward and backward using the undulating anal fin. Swimming behavior appears almost ghostly, with the fish maintaining a rigid body posture while the anal fin ripples continuously. Eigenmannia virescens is peaceful toward tankmates but may display mild territoriality toward conspecifics if space is insufficient. The species uses its weak electric organ to generate and sense electric fields, allowing navigation and prey detection in darkness or murky water. Stress or bright light causes the fish to hide for extended periods, while optimal conditions encourage more confident exploration and feeding activity.

Tank Mates

Selecting tankmates for Eigenmannia virescens requires caution due to the species' nocturnal habits, sensitivity to bright light, and slow feeding behavior. Suitable companions include peaceful, medium-sized community fish that are not overly boisterous or competitive during feeding. Potential candidates include larger tetras (Congo, silver dollar), peaceful Corydoras catfish, medium peaceful cichlids (Apistogramma, Geophagus), and other calm South American species. Avoid aggressive fish, fin-nippers, large predators that may consume the knifefish, and highly competitive feeders that prevent the knifefish from receiving adequate food. Very small fish may be consumed as the knifefish grows. Many experienced aquarists maintain glass knifefish in species-only or carefully curated South American biotope setups to eliminate stress and feeding competition.

Breeding

Breeding Eigenmannia virescens in home aquariums is uncommon and rarely documented. Successful spawning likely requires a large, mature tank with excellent water quality, soft and slightly acidic water, and a well-conditioned pair or group. Eggs are reportedly laid in sheltered areas, and fry require very small live foods such as infusoria or microworms initially. Raising fry demands exceptional water quality and specialized feeding regimes. Most specimens in the trade are wild-caught, with captive breeding remaining rare among hobbyists.

Why Buy from Tropical Fish Co

Tropical Fish Co supplies healthy, well-acclimated glass knifefish carefully selected for strong swimming behavior and good body condition. We ensure all specimens are feeding confidently on frozen or live foods before dispatch. Our expert team provides comprehensive care guidance covering the critical importance of dim lighting, secure hiding places, calm tankmates, and patient acclimation. With reliable UK delivery using specialist livestock couriers, your knifefish arrives safely in insulated packaging with oxygen. We support specialist fishkeeping and offer ongoing advice to help you succeed with this fascinating nocturnal species. Choose Tropical Fish Co for rare, quality livestock and expert customer support.