Paracheirodon axelrod, the Cardinal Tetrai

Paracheirodon axelrodi (Cardinal Tetra). The only question is, what's the second most glorious tetra, right? Their home waters in the upper Orinoco, Rio Vaupes and northern tributaries of Rio Negro aren't the tannic blackwaters of traditional aquarists' lore after all; instead fish hunters have found them in shaded clearwaters in the seasonally flooded forests. Softness counts when you're trying to please Cardinal Tetras, and the pH can drop below 6.0. The limestone waters of Florida will never suit them for breeding: in the U.S.A. these fish are still eluding the mass manufacturers and come in, wild-caught, somewhat seasonally, from the mid-reaches of the Rio Negro. 
 
Even at home, if you can't give them the very soft, acidic water they need, they'll never show their most intense color for you. Even if you forego the golden tones of peat filtration, some patches of shade from floating plants and a dark woody background, such as natural corkbark, present Cardinal Tetras at their best. A school is a must, or they'll be disoriented and shy.
 
These fishes first were first discovered in 1952 by Harold Sioli in a tributary of Rio Negro. In the winter of 1955-56 the first shipment of them was sent to New York, flown out from Manaus by the largest exporter, Paramount, who were pioneers in shipping Amazonian fishes by hydroplane. The new species was about to be formally named Hyphessobrycon cardinalis by Dr. George S. Myers (of Stanford) and Weitzman. But lo! the publication date of a newcomer magazine, Tropical Fish Hobbyist, was advanced by a few days, so that a rival scientific name honoring its publisher gained precedence. The ensuing dispute went all the way to the International Commission on Zoological Nomenclature before it was settled!
 
In Europe, the market is supplied by breeders in the Czech Republic. Wild stock for the U.S. market are still collected around Barcelos on the mid Rio Negro about 430 km from Manaus, and exported from Manaus, where the Rio Negro joins the Amazon. The fishing season coincides with low water levels from October to February. The Cardinal Tetra is the indicator species for the ornamental fish trade in the middle Rio Negro basin. There a thriving and sustainable ornamental fishery provides about 60% of the income of people living in the Rio Negro's floodplain. Of the 100 or so fishes in the trade, Cardinal Tetras constitute more than 80% of the total catch, some 30 million fishes every season, according to N.L. Chao, et al., Conservation and Management of Ornamental Fish Resources of the Rio Negro Basin, 2001  (proceedings of an international workshop on Amazon River Biodiversity at St. Louis, 1999). The fishery's importance to local people is a major support for the preservation of the old-growth rainforest in the region.
 
During the severe El Nino event of the 1990s, drought in the Rio Negro basin dried up populations of Cardinal Tetras in all the accessible creeks and igapes, causing some consternation in eco-circles, but after the rains returned, the natural populations bounced back in two seasons. The general lesson is, few tetras survive to maturity in nature. Cardinal Tetras are virtually annual fish, though they live for five years or so in an aquarium. Broader concerns, about the sustainability of the ornamental fishing trade founded on Cardinal Tetras and other piabas and  the social impact of the trade, are being studied by the Projeto Piaba, begun as a biological inquiry in 1989 but working now to develop a responsible sustainable Rio Negro ornamental fishery. You might be interested in reading about the Project and the Rio Negro fishing community at the Projeto Piaba website.
 
In Cardinal Tetras from the Rio Negro, the iridescent blue-green line ends at the base of the little adipose fin. In Rio Orinoco populations, the stripe ends short of the adipose fin, with a red streak that's noticeably wider in the caudal peduncle. In the Rio Negro there are also some "blond" or "golden" Cardinals, more curious than desirable, in my opinion.  
 
Besides Cardinal Tetras and Neons, a third look-alike tetra is Hyphessobrycon (Paracheirodon) simulans, rarely seen. But if you know what you're looking for, you might pick out a few as "contaminants" in a tank of imported Cardinals or Neons.
 
Paracheirodon axelrodi at FishBase. Paracheirodon axelrodi at Wikipedia.