Glass Knifefish Care Guide - Aquarium Setup, Diet & Breeding

Glass Knifefish have a long, laterally compressed body and a remarkable ribbon-like anal fin that extends along much of their underside. Their bodies are usually silvery, grey or translucent, with a darker line running along the back and a narrow, flexible tail region. The absence of dorsal and pelvic fins gives them an unusually streamlined appearance.

This species moves by gently rippling its anal fin, allowing it to glide forwards and backwards with impressive control. It is mainly nocturnal and often becomes most active after dark, when it explores the aquarium using a weak electric field to sense nearby objects and other fish.

In the wild, Glass Knifefish are associated with shaded, structurally complex habitats. Submerged roots, branches, aquatic vegetation and leaf litter provide shelter during daylight hours, while the fish venture out to search for small aquatic animals at night.

Eigenmannia virescens is a peaceful but specialised knifefish that can become a fascinating long-term aquarium inhabitant. It is social in nature and may show interesting interactions with members of its own species, particularly when kept in a suitably spacious environment.

Glass Knifefish are native to tropical South America, including parts of the Amazon, Orinoco and Paraná drainage systems. They inhabit slow-moving rivers, floodplain environments, forest streams and vegetated margins in countries such as Brazil, Peru, Colombia, Venezuela, Bolivia and Paraguay.

A group of adult Glass Knifefish needs a spacious aquarium of at least 400 litres, with a footprint of around 150 x 60 cm or larger; extra length and floor space are especially valuable. Use subdued lighting, gentle to moderate circulation and plenty of caves, tubes, driftwood, tall plants and shaded retreats. Leave open areas for swimming, while using a soft sand or fine rounded-gravel substrate that will not damage the fish as it searches near the bottom. A tightly covered aquarium is essential because startled knifefish may jump.

Glass Knifefish can reach approximately 45 cm (18 inches) in total length. Females may become slightly deeper-bodied, while overall maximum length is broadly similar between the sexes.

Glass Knifefish are carnivorous and prefer meaty foods. Offer a varied diet of frozen or live bloodworm, blackworm, brine shrimp, mysis shrimp, chopped earthworm and other appropriately sized aquatic invertebrates. They usually feed most confidently in the evening or after the aquarium lights have dimmed, and high-quality sinking carnivore pellets can be used as part of a varied routine once the fish is settled.

Glass Knifefish can live with peaceful, similarly sized South American fish such as larger tetras, peaceful cichlids, hatchetfish, larger Corydoras, peaceful loricariid catfish and other non-aggressive community species. Avoid boisterous or fin-nipping fish, aggressive cichlids and species that compete heavily for shelter. Very small fish, shrimp and delicate fry may be eaten, especially once the knifefish reaches adult size.

Glass Knifefish are difficult to breed in home aquariums. They are egg scatterers, and spawning is associated with seasonal changes in water conditions, including shifts in temperature, water level and flow. A large, mature aquarium with excellent water quality, dense cover and a compatible group may encourage spawning, but successfully raising the young requires careful separation from the adults and very small live foods.

Visible differences between male and female Glass Knifefish are subtle. Mature females are often fuller through the abdomen, particularly when carrying eggs, whereas males may appear slightly slimmer; reliable sexing is difficult outside the breeding season.