"Hyphessobrycon" herbertaxelrodi, the Black Neon Tetra
"Hyphessobrycon" herbertaxelrodi (Black Neon). I've kept Black Neons in my somewhat generic "Amazon" aquaria, even though they aren't fishes of the Amazon Basin; in fact, Black Neons come from the Mato Grosso, far to the south in Brazil, in the Rio Taquari, which is part of the Paraguay drainage. The area was for long inaccessible even to a cargo plane, so remote that "Hyphessobrycon" herbertaxelrodi didn't arrive in the U.S.A. until 1960; the following year the Characid expert Dr Jacques Géry gave it a clunky name honoring Dr. Axelrod that set off a tiresome modern fad for flattering v.i.p.s with attaching their complete names to new species: why not "doctorherbertraxelrodi" while you're at it, eh? In spite of what you might be told in books like B. Ward's Aquarium Fish Survival Manual, Dr Axeldrod was not actually the "discoverer".
When they were first imported, Black Neons seemed to require soft water and peat filtration, but they have settled down in captivity and have become more tolerant of a wider range of water parameters. Any that you may see today are captive bred, and no one thinks of Black Neons as especially delicate and demanding tetras any more. They have become naturalized near Hong Kong, where they were reported in 2001 in a stream draining Xiaowutongshan during a biodiversity survey in Wutongshan National Forest Park.
In the aquarium, if they are well-fed, and not too jostled and distracted by other species and if water is soft and pH is slightly acidic, Black Neons will spawn casually in a community tank. Peat filtration will help. You'll see them sporting and chasing in the evening, and in the morning, unless you're up at dawn, it will all be over: the females will be slender once more and the eggs will have all disappeared.
Though they tend to shoal together, a slight degree of intra-specific aggression keeps H. herbertaxelrodi from acting in unison as a true school. I find they like to maintain a station briefly near a leaf.
A disappearing act: the patterns and the colors or transparency of fishes is never arbitrary. We know that selection by predators, which helps enforce uniformity within the shoal, and by sexual attraction have shaped these animals. I was vividly reminded of the survival value of Black Neons' patterning one afternoon when I was watching them dashing back and forth in a shaft of sunlight. The upper body of this fish is all but transparent. The bold iridescent green stripe down the center of the fishes' flank marks a sharp upper edge to a black streak that counter-shades away below to a silvery white belly. I suddenly compared the fish in sunlight to a small shiny horizontal leaf near it, with a blinding band of sunlight across it and its top facing me in shadow. How unfishlike the camouflaged Black Neon appears. Then it turns away to flee and disappears, like a deep-keeled cardboard fish-shaped cutout that's suddenly been turned end-on! What is a predatory fish presented with? — a flash of irridescent green, like a leaf surface caught in sunlight, then a snap disappearance!
Every scientist agrees that the genus "Hyphessobrycon" is actually a mixed bag of characids, with variable chromosome numbers: for instance, in 1990 V.A. Arefjev established that "H." herbeertaxelrodi is 2n=52. Until a revised analysis is published, it might be best to keep the genus in quotes, as we do with "Cichlasoma".
Links. There's a good species profile at TropicalFishkeeping, with a glimpse of the current state of uncertainty and disorder in characiform taxonomy.
Species profile of Hyphessobrycon herbertaxelrodi at Tropical Fish Hobbyist. Hyphessobrycon herbertaxelrodi at FishBase. Hyphessobrycon herbertaxelrodi at Wikipedia.