Family Characidae or American Tetras
Incertae Sedis Group
Genus Gymnocorymbus.
— Bondi Tetra — Gymnocorymbus bondi [Fowler, 1911]. Synonyms: Gymnocorymbus socolofi, Phenacogaster, Moenkhausia bondi. Venezuela, Colombia, Orinoco River basin. 5 cm. Olive-beige with a light green stripe along its deep body and a black mark at the base of the caudal peduncle. Unpaired fins in juveniles are reddish, but fade with age. Males are brighter and smaller, with a concave anal fin.
Needs an aquarium planted along the edges, floating plants, and a dark substrate. Can be kept in a community tank in a school of at least 6 individuals, with thickets of vegetation in places. Do not keep them with veil-tailed fishes or labyrinth fish with long pelvic fins, as they are prone to fin-nipping. Omnivorous; they take all standard aquarium foods, as well as live and frozen items. Supplementary plant matter is required, otherwise they will nibble on soft-leaved plants. Temperature 22-25°C; dH 5-20°; pH 5.5-6.5.
— Black Skirt Tetra, Black Widow Tetra — Gymnocorymbus ternetzi [Boulenger, 1895]. Synonym: Tetragonopterus ternetzi. (Eng: Black Skirt Tetra, Black Widow Tetra, Butterfly Tetra, Goldskirt Tetra, Petticoat Tetra, Strawberry Tetra, White Skirt Tetra). Bolivia, Peru, Brazil, Paraguay; Paraguay, Matos, and Guaporé river basins. 5-6 cm. The fish has a very deep, diamond-shaped, and strongly laterally compressed body with high dorsal and broad anal fins. The anal fin starts from the middle of the abdomen and extends to the caudal peduncle.
Juvenile coloration is dark, smoky-black with a yellowish to bluish sheen; the black bars have a stronger luster. The grey background is crossed by 3 vertical black bars: one through the eye, the second behind the operculum, and the third below the dorsal fin. The anterior part of the body is lighter and silvery. Pectoral and caudal fins are transparent, altering the fish’s optical profile. Dorsal and anal fins are black; pectoral fins are translucent white. The adipose and caudal fins are transparent, the former with a black margin, and the latter deeply forked with white tips in males.
The entire posterior section of the body, the tall dorsal fin, and the very broad anal fin are jet-black, creating the appearance that the fish is wearing a dark skirt. With age, the coloration lightens, dark markings lose their sharp borders, and scales resemble aged tarnished silver; however, during breeding, intense juvenile shades return. If such fading occurs in young individuals, it is often a sign of disease or stress.
Males are smaller, with whitish tips on the caudal fin lobes. The dorsal fin is narrower, while the anal fin is noticeably broader in its anterior part, with an arched lower edge. Females have more black coloration and are larger and rounder. When viewed from above, the female is visibly broader than the male. In the aquarium hobby, long-finned varieties are also bred: “Longfin Blackskirt Tetra”, “Long Finned Black Widow”. Yellow and albino strains have also been developed: “Gold Skirt Tetra”, “White Skirt Tetra”, “Gold Widow Tetra”. Artificially dyed specimens in pink, orange, green, or blue shades (“Colored Skirt Tetra”, “Colored White Skirt Tetra”) also enter the trade; their coloration usually fades within 6 months.
Identification: Butterfly Tetra — Gymnocorymbus ternetzi.
It is best to purchase Black Skirt Tetras when young, at 2-3 months of age, in a group of at least 6 individuals. At this stage they are more active and display their deepest coloration: dark grey tones turn coal-black under optimal conditions and good health. Upon reaching old age (up to four years in aquariums), they become sluggish, and even non-predatory tankmates may readily nip at their prominent fins.
This hardy and lively fish requires a spacious aquarium where open swimming areas alternate with dense plant clusters. They move in small groups, frequently sparring with one another, and can show mild aggression toward smaller fish. In large aquariums, they prefer middle depths in open water, while in small tanks they hide among vegetation, emerging only to feed.
When kept alone, they often turn aggressive. In a school, you will observe males competing for female attention and establishing a loose hierarchy. Thanks to their generally peaceful disposition, they can be housed with other schooling fishes, making them ideal for community setups. The water should be clean and peat-filtered, with weekly water changes. Water chemistry is flexible; soft to moderately hard water with a slightly acidic to neutral pH is preferred.
Equipped with an upturned mouth, they feed mainly near the surface, eagerly accepting any live, frozen, or dry food. Foraging from the bottom is difficult for them. They reach sexual maturity at 8 months of age and breed readily if the aquarium provides sufficient live plants. Lifespan is up to 4 years. An excellent choice for beginners: robust, undemanding, and easy to breed. pH 6.0-7.5; dH 4-18°; Temperature 23-26°C.
Identification: Black Skirt Tetra — Gymnocorymbus ternetzi.
— False Black Tetra — Gymnocorymbus thayeri [Eigenmann, 1908]. Synonym: Moenkhausia profunda. (Eng: False Black Tetra, Falso Tetra Negro). South America: Bolivia, Colombia, Trinidad and Tobago, Amazon and Orinoco river basins. 6 cm; females are larger. The deep, silvery-reflective, almost disc-shaped body shines bluish or violet depending on the incident light, most noticeably on the upper half. Marked with a blue stripe along the lateral line and two faint vertical dark bars behind the gill cover and beneath the dorsal fin. Fins are colorless, occasionally with a grayish tinge. It is essentially a paler version of G. ternetzi without bold vertical bars, featuring a more convex rather than rounded anal fin margin.
An active, fast-swimming schooling fish. Compared to G. ternetzi, it is significantly shyer and less robust. Suitable for community aquariums alongside similarly peaceful, larger, and active schooling species. It may show minor aggression toward smaller fishes. Dense, fine-leaved plants should be placed along the margins, leaving ample open swimming space. May nip at tender young plant shoots. Water should be soft and slightly acidic. Voracious feeder; accepts live, vegetable, and dry foods readily. Can be bred in roomy aquariums. Lifespan is up to 7 years. pH 5.5-7.5; dH 4-18°; Temperature 19-24°C.
Hemigrammus bleheri, Hemigrammus coeruleus, Hemigrammus erythrozonus.
Astyanax kennedyi, Astyanax leopoldi, Astyanax mexicanus. 
