Photo by Leo G. Nico and taken from Piranhas in the Aquarium, by Wolfgang Schulte.

Serrasalmus Niger: The black piranha.  A rare species, it is regarded as the most dangerous and unpredictable of all piranhas.  Young fish are light silver in color and the dark grey to black is not achieved until late adulthood.  Maximum length is up to 35cm (14 inches).  It likes poorly lit conditions with plenty of cover.  In the home aquarium, it is strongly recommended to have only one fish.  In close quarters this species is much more likely to turn cannibalistic even if well fed.
     This piranha's notorious behavior has been fairly evident in my own aquarium specimen.  It shreds all vegatation including the plastic variety.  I cannot have anything but bogwood or rock as decoration.  My computer desk is in the same room, and the piranha appears to "watch" me as I sit at the desk.  I say this because it will consistently stay in a corner closest to me. I can wave my hand around attempting a poor representation of another fish and my piranha will watch and follow the movement.  It otherwise swims all around the 55 gallon tank when unaware of my presence.  I do not have a picture of my own fish but plan to do so in the near future.