Members of the family Characidae share many characteristics with other freshwater fish. They are a member of the Ostariophysi super order, which includes around 68% of all freshwater species in the world. This super order is characterized by the Weberian apparatus and some sort of alarm substance that is released when the fish is stressed. Family Characidae were originally discovered in South and Central America, but have also been found in central Africa.
There are two main variations of Characidae: carnivorous and omnivorous. The omnivorous kinds will eat both vegetable matter and meat, while the carnivorous only eat meats. While there are also herbivorous Characidae, you should probably avoid keeping them, because they will eat literally every plant in your aquarium.
When choosing to keep carnivorous Characidae, there are some things you will want to keep in mind. First and foremost, be aware that anything smaller than the Characidae will be considered food. And mixing other aggressive carnivores will only lead to one or the other being eaten. You will want to keep herbivorous fish that are much larger with Characidae. They will generally leave anything bigger than themselves alone, and the herbivore will not be interested in attacking unless provoked. In contrast to the piranha, which has similar jaws and eating habits, a Characidae will attack both alone and in schools. Piranhas tend to attack only in schools. But the effect either type can have on your hand is similar, so be careful. These kinds of fish are definitely not recommended for the beginner.
Carnivorous Characidae will eat pretty much any kind of meat. Worms, smaller fish (a lot of dangerous fish keepers will stock the aquarium with several small, cheap fish so the predator has something to hunt), etc. Try to avoid giving them a taste for red meat, and do not put your hands into the tank while feeding them.
Characidae live in the brackish waters of South and Central American rivers. Lots of peat and vegetable matter contribute to hazy waters and a PH leaning towards acidic. When properly kept, Characidae will breed in captivity, leaving eggs that stick to the bottom of the aquarium.
They will eat their young however, so if you are going to attempt to breed Characidae be prepared to move either the adults or the young into a separate aquarium right after hatching.
Serrasalmus Rhombeus, also know as white piranha or spotted piranha, are another favorite of people who like to keep dangerous fish. Like Characidae, piranhas are from South America, in the Amazon Basin. They can grow up to 10 inches long in the wild, but most aquarium-raised piranha only grows to around 6 inches. As with Characidae, piranhas are not for the beginner.
As one of the most aggressive fishes, they cannot be kept with other fish. Their most prominent feature is the over-sized jaw, with rows of needle-sharp teeth. Piranhas have been known to tear apart aquariums with those teeth. When netting a piranha, you must use extreme caution to keep your hands away from the fish at all times. They will attack and shred your hand without hesitation if you let them. In the wild, a school of piranha can reduce an entire cow to bones in a matter of seconds, so your hand is just a little snack for them.
Piranhas eat meat, of course. They will attack and eat any other fish they are kept with. Some of the nicer things about keeping piranha include their relative indifference to water conditions, and that they will not breed in a tank. This prevents schools of piranha from appearing in your tanks.
They will eat each other in captivity, especially if there are no other fish to attack. Piranhas are extremely aggressive and dangerous, and should only be kept by experienced keepers.