This article is from the Aquaria: Good (and Bad) First Fish; Breeding FAQ, by Dean Hougen and Elaine Thompson
Piranhas are among the most abused of all aquarium fish. They are
often purchased in order to watch their legendary feeding habits. As
mentioned above, feeder fish often bring diseases and parasites with
them and these can infect Piranhas. A regular diet of feeder fish can
also be quite expensive.
Piranhas are schooling fish and are generally shy and stressed when
kept as single specimens. Unfortunately, they also get big (many
species well over a foot long), so most beginning aquarists don't have
room to house more than a single Piranha. If enough tank space is
available to keep several Piranhas together, they must be kept well
fed or they will turn on each other, killing and cannibalizing one
fish after another.
 
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