Paracheirodon is a genus of small characins (family Characidae) from the blackwater and clearwater tributaries of the Amazon and Orinoco basins, with all three atlas species - the cardinal tetra (P. axelrodi), the neon tetra (P. innesi), and the green neon tetra (P. simulans) - native to Brazil, Colombia, Venezuela, and Peru. They are slim-bodied schooling fish under 1 in, unified by an iridescent blue-green lateral stripe that runs the length of the body and reflects light even in the tannin-stained, soft, acidic water of their native streams, where visibility is often just a few centimetres - the stripe is thought to help shoal-mates keep together in that gloom.
Few fish are more central to the freshwater aquarium hobby than the neon and cardinal tetra: both are among the best-selling ornamental fish in the world, and the sustainable wild harvest of cardinal tetras from the Rio Negro basin underpins a well-documented conservation-through-commerce economy for local fishing communities in that region. All three species keep well in soft, slightly acidic water in a shoal of six or more, and the green neon, while less commercially dominant, is a favourite among blackwater-tank specialists for its smaller size and cooler blue-green tone. Two of the three atlas species are assessed as Least Concern; the third has not yet been evaluated.