Paracheirodon axelrodi, or Cardinal Tetra, is a beauty.  These fish are from the Amazon, and are typically found in small, soft-water, rivulets and pools in the jungle.  I know of no freshwater fish more colorful and beautiful than the Cardinal Tetra.  Their coloration is extremely striking, especially when a school of a dozen or more make themselves visible.  I made the mistake of keeping this fish with Angle fish, under the assumption that if the Angels and the Cardinals matured in the same tank, the Angels would not bother the Cardinals.  This turned out not to be the case.  As a result, although I no longer keep Angel fish with the Cardinals, my Cardinals have learned to stay in the shadows of my  deeply planted tank.  I almost never see these fish toward the front of the tank.  They are much more comfortable swimming within dense Valisneria growth.  If you plan to keep this fish, never keep them with Angel fish.  Angel fish are their natural predator, and will eventually harass this fish into potentially permanent hiding. 

Cardinals are a bit fragile, and should be treated delicately when relocating.  They prefer temperatures of between 72 and 78 degrees Fahrenheit, but will tolerate higher temperatures.  Many people will keep Cardinals with Discus fish, in which case the tank temperature may exceed 85 degrees Fahrenheit.  Once established, the Cardinal Tetra is an easy fish maintain.  They happily eat dried and frozen foods.  They should be kept with six or more of their kind, since they are a naturally schooling fish.

Both males and females look identical, though it is reported that the location of the rear part of the swim bladder (which can be seen when the fish is viewed against a strong light) is pointed and curves down sharply towards the anus.  In the female, the swim bladder is more evenly shaped, and the space between the swim bladder and the internal organs is occupied by the ovary, which shows up as a dark shadow.