Knife Livebearer Care Guide - Aquarium Setup, Diet & Breeding

The Knife Livebearer is a distinctive poeciliid named for the sharp, blade-like profile formed by the lower edge of its body and anal region. Its elongated body is usually silvery to olive-brown, often with a darker lateral marking, while the fins may show subtle yellow, amber or dusky tones. It is a streamlined and attractive livebearer rather than a brightly coloured one.

Knife Livebearers are native to Central America, occurring mainly in Costa Rica and Panama, with records also associated with neighbouring parts of Nicaragua. They inhabit freshwater streams, creeks and rivers in humid tropical landscapes.

A group of Knife Livebearers should have an aquarium of at least 80 litres, with a footprint of roughly 90 cm by 30 cm to provide swimming space. Use gentle to moderate flow, efficient filtration and good oxygenation, reflecting their natural stream habitat. A dark sand or fine-gravel substrate, driftwood, smooth stones and dense planting around the sides and rear create a secure, natural-looking layout while leaving open water for activity. Floating plants can provide useful cover for females and fry.

Females can reach about 10 cm (4 inches), while males are usually smaller at around 7.5 cm (3 inches).

Offer a varied omnivorous diet consisting of quality flakes or small pellets, supplemented with frozen or live foods such as brine shrimp, daphnia and mosquito larvae. They also graze on algae and other plant material, so spirulina-based foods and blanched vegetables can be useful additions. Feed small portions regularly and ensure that smaller fry receive suitably fine food.

Knife Livebearers are generally peaceful and can be kept with similarly sized, calm community fish such as peaceful tetras, rasboras, small barbs, Corydoras and other non-aggressive livebearers. Keep them in a group, as males may court females persistently. Avoid large predatory fish, fin-nippers and boisterous species, as well as very slow-moving fish that may be stressed by their active behaviour.

Knife Livebearers are livebearers and will breed readily when well fed and maintained in suitable water. Males fertilise females using their gonopodium, and a female typically gives birth to a small brood of free-swimming fry after a gestation period of around four weeks. Provide dense plants such as Java moss or floating cover so the young can escape predation, and remove or separate adults if a high survival rate is desired. Feed the fry newly hatched brine shrimp, finely powdered foods and other appropriately sized offerings.

Females are larger and deeper-bodied than males. Mature males possess a modified anal fin called a gonopodium, used to transfer sperm, while females retain a broad, conventional anal fin and may develop a visible gravid spot when carrying fry.