Knife livebearer

Alfaro cultratus

FreshwaterFish

Care at a glance

Temperature: 75–82°F (24–28°C)
pH: 6.5–7.5
Adult size: 3 in (7.6 cm)
Minimum tank: 32 gal (120 L)
Temperament: Mostly peaceful
Social behavior: Social
Swim level: Middle
Difficulty: Difficult
Breeding: Medium
Rarity: Rare
Origin: Costa Rica, Nicaragua, Panama
Family: Poeciliidae

Overview

The knife livebearer is a slim, silvery fish that lives in the middle and upper water and does best in a group. It is mostly peaceful toward other species, but males squabble among themselves, so keep it either as a small single-species group with room to spread out or alongside calm, similar-sized fish. It can be shy and is a little demanding about clean water, so give it steady filtration and gentle current. Feed a varied mix of flake, frozen and live foods. It likes soft to medium, near-neutral water. A tank of 120 liters or larger gives a group the space it needs.

In the wild

Alfaro cultratus comes from the warm lowland streams and rivers of Costa Rica, Nicaragua and Panama, where it lives in flowing, well-oxygenated water. Keep it between 24 and 28 C with a pH from about 6.5 to 7.5 and soft to moderately hard water. A planted tank with clear swimming lanes, gentle to moderate flow and very clean water recreates its home. Stable conditions and good oxygen keep this slightly sensitive fish comfortable.

Good to know

The knife livebearer belongs to the family Poeciliidae and grows to around 7 to 8 cm. It is named for a keel of enlarged, overlapping scales along the lower edge of the tail, which gives the body a sharp, knife-like underside. Males carry a gonopodium and long, extended pelvic fins, and they fertilize females internally so that the larger, fuller females give birth to live young. Breeding is possible at home but takes some care, earning it a moderate rating.

Did you know

Studies have found that knife livebearers from streams full of predators grow slimmer, more knife-edged bodies than those from safer waters, a shape that helps them dart away from danger.