Order Cyprinodontiformes
Suborder Cyprinodontoidei
Family Poeciliidae
Subfamily Poeciliinae
Genus Alfaro
These fish are of little significance for aquarium keeping. An interesting biological feature of the genus is the presence of right- and left-handed gonopodia in males. In this regard, the possibility of obtaining offspring is determined by the compatibility of the breeders: males and females can be either “left” or “right”.
— Knife Livebearer — Alfaro cultratus [Regan, 1908]. Synonyms: Alfaro acutiventralis, amazonum; Petalosoma, Petalurichthys amazonum, cultratum. English: Knife Livebearer, Knife Edged Livebearer. Nicaragua, Panama, Costa Rica. A larger population is widespread in Central America: females up to 9 cm, males up to 7 cm; the northern part of the Amazon basin is inhabited by Alfaro whose length does not exceed 6 cm.
The body is elongated, quite high, laterally compressed, especially in the caudal peduncle area. The eyes have a blue or turquoise iris. The body is transparent, the vertebral column and swim bladder are clearly visible through it. The coal-black pupils of the eyes seem to be enclosed in an emerald frame. The main body coloration ranges from light to olive-green, strewn with turquoise scales that sparkle in reflected light.
Fins range from colorless to yellowish-green. The dorsal fin is dotted with small dark spots. The caudal fin has a dark border. Along the lower edge of the body behind the anal fin, there is a knife-shaped outgrowth formed by scales. Males are smaller, more brightly colored, and possess a gonopodium directed to the right or left side.
These are very agile, schooling fish that swim quickly and are capable of jumping out of the water. A spacious aquarium with aeration, filtration, and water changes is preferred. It is advisable to keep a school of 8-10 fish, with a ratio of 2-3 females per male. They can be kept in a community aquarium with other peaceful fish, well-planted with fine-leaved and floating plants, while having open swimming space in the upper layer.
Somewhat aggressive fish, occupying the upper level of the aquarium, preferring a current. In general, these fish are quite suitable for keeping in community aquariums, but they are easily injured in skirmishes with neighbors (bacterial infections). Good water filtration and aeration, as well as weekly water changes, are necessary. The fish prefer slightly salted water at a concentration of 0.5-3 g/liter. When adding salt, it must be taken into account that not all species of fish and plants may like brackish water.
Adult fish prefer live foods (daphnia, tubifex, corethra) and flake vegetable foods. They do not take food from the bottom. The fish are very sensitive to bacterial diseases. They mature at 5-7 months. Eggs are fertilized inside the female’s body, and fully formed fry are released, which immediately begin to take food.
A group of fish is planted for spawning because, due to anatomical differences (left and right placement of reproductive organs), not every male is suitable for every female. Depending on water temperature, female pregnancy lasts 6-8 weeks. The fish do not pursue the fry. Lifespan is up to 3.5 years. pH 6.5-8.0; dH 5-12°; T 24-28°C; salinity 1-5%.
Identification: Alfaro cultratus.
— Huber’s Knife Livebearer — Alfaro huberi [Fowler, 1923]. Synonyms: Furcipenis, Priapichthys huberi. English: Huber’s Knife Livebearer. Nicaragua, Guatemala, Honduras. Male 4 cm, female 7 cm. Olive-gray with a dark zigzag border on the scales and a black spot in the middle of the body. Males are smaller, pelvic fins are elongated and feature a gonopodium. Eyes are turquoise, fins are yellowish.
It is advisable to keep them in a school in an aquarium with dense vegetation, driftwood, and smooth stones with proportionally sized fish. Floating plants are necessary. They can be aggressive, especially towards males of their own species. Weekly water changes are mandatory. They mature at 5 months. Pregnancy lasts 3-4 weeks and up to 25 fry are born. pH 7-8; dH up to 10°; T 24-28°C.
Brachyrhaphis cascajalensis, episcope, hartwegi, holdridgei.
Scolichthys greenwayi, Tomeurus gracilis, Xenophallus umbratilis. 
