• Scientific name: Alfaro cultratus
• Synonyms: Alfaro acutiventralis, Alfaro amazonum, Petalosoma amazonum, Petalosoma cultratum, Petalurichthys amazonum, Petalurichthys cultratus
• Common name: Knife Livebearer, Knife-edged Livebearer
• Group: Livebearers
• Habitat: Central America; Costa Rica, Nicaragua and Panama
• Size: 7.5 cm
• Biotope: Inhabits in fast-flowing rain forest streams.
• Social behavior: Can be aggressive with conspecifics, so it is best to keep them in an aquarium with their own species.
• Diet: Omnivorous; Unfussy, eats everything from live foods to flakes.
• Breeding: Easy
• Tank: Minimum 80 litres
• Population: 2 pairs for 100 litre
• Decoration: Needs a tank with dense vegetation and open areas for swimming. Decorate the tank with roots and rocks. A strong filter and a regular changing of the water is necessary.
• Temperature: 24-28 °C
• pH: 6-8
• Hardness: 5-20 NK°
• Lifespan: 3-5 years

Description: Knife Livebearer is a hardy and adaptable species. This fish occupies the upper level of the aquarium, adapted to feeding on insects from the surface. Their body is yellowish, with a blue iridescence on the flanks. Their flattened bodies allow them to swim through strong water currents with less friction. The fish is very sensitive to bacteria. The vental side of this fish has a knife-like keel. This keel is formed by two closely stacked rows of scales at the posterior end of the body.

The male has a gonopodium and extended ventral fins which serve to stimulate the head region of the female during pairing rituals. Females are larger and plumper. Breeding is easy. Every 3-4 weeks the female can produce 20-40 fry in a single brood. Fry should be removed to a separate tank, and fed with small live foods or powdered flake. The fry are elongated, transparent, and barely visible, with a length of about 6mm. About 6 months of age, the animals become sexually mature.