Cardinal tetras mainly communicate with body motions. When there’s a disagreement, the parties involved will hold their fins rigidly, which gets their message across.
Cardinal tetras aren’t good on their own, and it’s best to have a group of them live together. If they’re alone for too long, they may get sad or depressed and put their health at risk. Still, if you have the space, they’re one of the prettiest, best pet fish you can keep.