The cardinal tetra is an amazonic fish related to the piranhas. They're very small and they aren't so aggressive, so they're not a problem for us. They live in water between 23ºC and 27ºC (73ºF and 80ºF), with a pH between 4,6 and 7. The amazonic water has a low pH due to the high amount of decomposing organic material such as tannins, phenols, humic acids from the vast vegetation. The cardinal fish originated in Brasil, in the 'Rio Negro' and its affluents.
The cardinal tetra is a group animal and they usually swim in groups of, at least, 4-5 fishes. They're usually about 5 centimetres long.
Finally, Cardinal tetra has a really similar brother, it's called neon. As you can see in the photo, the cardinal has a fully complete side to side red and blue stripes, but the neon hasn't. Also, a tetra cardinal in captivity reaches 4 - 5 cms, while the neons stay at 3 - 4 cms.
Thanks,
Boll