Probably has the reputation as one of the most aggressive and most dangerous Piranha’s to keep. Serrasalmus Rhombeus (Niger) or Black Piranha probably comes close to that reputation, no tank mates are safe from this fella and extra care should be taken when cleaning. They reach a size of approx. 15-18 inches so they should have a fairly decent size aquarium. We have had them in a few different colour varieties, so they are not necessarily black in colour, juveniles are typically silver and have black eyes until they reach a good 5-6 inches when the eyes then turn red and body becomes dark in colour. As they mature they can get very in dark in colour and super red eyes and a high back...actually a very nice looking fish.