The black piranha (Serrasalmus rhombeus) is a shy, solitary fish. A shy, solitary fish, mind you, with extremely strong jaws. He’s best kept alone; other fish will stress him and he might eat them, and it’s not really a good idea to habitually dunk your fingers in the tank.
Although they grow slowly, black piranhas can reach a length of at least 16 inches and so need a very large tank -- 100 gallons is about the minimum. You’ll also need all the usual items for a freshwater tropical aquarium, including a light, heater, aquarium gravel, aquatic plants and a powerful filter. Bogwood and rocks make suitable decorations and hiding places.
Black piranhas prefer soft, slight acidic water with a pH somewhere between 5.8 and 6.8. Maintain a temperature of about 75 to 80 degrees Fahrenheit. They do not like powerful light so use a low-intensity bulb, enough to keep any plants alive but not so bright that it stresses the fish.
The short answer to the question -- “What are suitable tank mates for the black piranha?” -- is none, not even the same species. If you plan to breed, you’ll need more than one tank. Black piranhas are not suited to any sort of community aquarium. Larger fish will cause stress, no matter how peaceful their disposition, and he might attack them. Smaller fish will probably become lunch. If you are breeding them, remove the female after spawning. The male looks after the eggs but should also be transferred to another tank shortly after they hatch.
Piranhas deserve their voracious reputation. They are carnivores with an unselective diet, pretty much eating anything that swims past. The black piranha tends to go for a diet of invertebrates and smaller fish, rather than bits of larger animals. In captivity, his diet should consist of live food such as earthworms, brine shrimp and bloodworms. Some individuals will take bits of meat or (dead) fish after a while, and some even learn to accept flakes. He still, however, needs a varied diet that includes some live food.
This is certainly not a fish for beginners. Experienced aquarists should have few problems but bear in mind that ownership, transport and sale of this species is banned in many places, including much of the United States. This is because accidental or deliberate release of piranhas endangers native wildlife and commercial fisheries.