iv heard that too but still have yet to see it in my tank or in the my sisters friend has. maybe im just getting the more mellow fish.
Read this article -
http/network.bestfriends.org/friends_of_.../news/3672.html
It discusses goldfish and their needs and touches on highly aggressive goldfish, etc etc.
This one, too.
http/network.bestfriends.org/friends_of_...news/13273.html
"Myth: It is OK to keep fancy and single tail varieties together.
Reality: While it is usually safe to keep small, young single-tail goldfish with fancy varieties, it should never be a long-term situation. Fancy goldfish have delicate and often inhibited bodies, as well as very sensitive swim bladders, and can easily be injured by the faster, more
aggressive commons and single-tails. Single-tail goldfish are also incompatible as they often require more space (such as a pond!) and lower temperature ranges."
"Myth: Goldfish are peaceful and make great (tropical) community fish!
Reality: It is true that goldfish make great community fish... if it is a community of their own species, and perhaps one or two other compatible species. However, goldfish are neither entirely peaceful nor great mixed-species community fish! There are a number of reasons for this. First of all, goldfish are massive waste producers, and can make other fish ill with their waste output. Secondly, this high waste demands massive filtration and aeration, which rules out many species that are ill suited for extreme currents. Thirdly, goldfish are a cold water species, and most commonly kept aquarium fish are tropicals; even fancies, who prefer slightly higher temps, are normally kept on the low end of the health and comfort range for tropical fish. Fourth, goldfish grow much larger than most community fish, even other cold water fish, and will eat anything that fits in their mouth... you do the math! Fifth, goldfish love to eat aquarium plants, yet many sensitive species do not thrive with plastic plants. Sixth, goldfish are known for nipping fins and aggressively head-ramming and body-slamming (especially singletail varieties); they can and will hurt more delicate fish if vying for food or in the mood the mate. Seventh, fancy goldfish often have swimming problems, protruding eyes, or other physical inhibitions and may be picked on by swifter, more aggressive tank mates. Eighth, goldfish are gluttons and will not eat just their food, but everyone else in the aquarium's too! Ninth, goldfish are hosts to many internal parasites that do not affect them, but can be deadly to more sensitive species (ie. flukes). Tenth... well, you get the picture!"
I promise I wasn't lyin to ya. Lol.
They're a member of the carp species, who are semi aggressive in their own right. They can bully and eat other membes of their own species out of territorial aggression.