Fish
Cardinal Tetra
Paracheirodon axelrodi · also called cardinal tetras
A brilliant red stripe runs the full length of a cardinal tetra’s body, topped by an iridescent blue-green line, as it schools tightly with dozens of others through the dark, tannin-stained waters of an Amazon blackwater river.
Overview
The cardinal tetra, Paracheirodon axelrodi, is a Least Concern fish native to blackwater rivers of the Amazon basin, particularly the Rio Negro and Orinoco. Its full-length red stripe, strict schooling behaviour, and adaptation to soft, acidic blackwater make it one of the most popular and visually striking small fish in the aquarium hobby.
A stripe that tells the species apart
While often confused with the closely related neon tetra, the cardinal tetra’s red stripe runs the entire length of its body rather than stopping halfway, a simple but reliable way to distinguish the two similar-looking species at a glance.
Status
Least Concern, remaining widespread across suitable blackwater habitat in the Amazon basin, with large numbers also captive-bred for the aquarium trade.
Cardinal Tetra: common questions
How can you tell a cardinal tetra from a neon tetra?
The most reliable difference is the red stripe. On a cardinal tetra the red runs the entire length of the body, from the gills to the tail, while on a neon tetra the red only covers the back half of the body. Cardinals are also slightly larger and come mainly from the Rio Negro and Orinoco, while neons come from the upper Amazon.
Why does the cardinal tetra need to be kept in a group?
It is a strict schooling fish that in the wild lives in groups of hundreds, and when kept alone or in very small numbers it becomes timid, hides, and loses its vivid colour, so a proper school of at least six, ideally more, is needed for it to display natural behaviour and health.
Why does the cardinal tetra need such specific water conditions?
It evolved in blackwater rivers stained dark by dissolved plant tannins, with naturally soft and acidic water, and it does best in an aquarium that replicates those conditions rather than harder, more alkaline water.
Is the cardinal tetra hardier than the neon tetra?
It is generally considered somewhat more robust and more resistant to a common tetra disease than the neon tetra, making it a slightly more forgiving choice for some aquarium keepers.