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First off, let's pronounce the name right. It is not "piranna,", as most English-speaking people tend to mispronounce it. We should learn right now that the proper pronunciation is close to the English "peer-on-yah." Is that too hard to say?
The name piranha is from the great general South American Indian language called Tupi-Guarani (pronounced too-pee gwah-rah-nee).

A fish only a foot long with teeth so sharp and jaws so strong that it can chop out a piece of flesh from a man or an alligator as neatly as a razor, or clip off a finger or a toe, bone and all, with the dispatch of a meat-cleaver! A fish afraid of nothing, which attacks any animal, whatever its size, like lightning! A fish which never attacks singly but always in schools of a hundred or a thousand! A fish which is actually attracted by splashing and commotion in the water! And a fish which, when it smells blood, turns into a raging demon! This is the piranha, feared as no other animal is feared throughout the whole length of South America.
The piranha is the most dangerous fish in the Amazon and perhaps in the world. That evil reputation has for a long time rested with the Great White Shark (Carcharodon), but the swimmer menaced by that 30-foot man-eater usually has only one animal to fight off and agility or a lucky poke in the eye of the great fish might save him. The swimmer menaced by a school of lightning-swift piranhas would have no such chance, for nothing he could do would drive them off once he had been attacked. Moreover, if we care to contemplate preferences in regard to the manner of meeting are end, most of us would choose the single snap of the shark instead of being chopped to pieces by a hundred razor like sets of teeth!.

The teeth and jaws with which the piranha can do such execution have been described many times, so I need not give too long a description. The jaws of the most dangerous species are short and broad, and the lower one is deep and powerful. The teeth are triangular in form, with an acute median point and very sharp edges, and each bears a small but sharp-edged cusp on the posterior side. This cusp is usually in line with the anterior sharp edge of the succeeding tooth, and blends with its profile, so that the row of teeth looks, in profile, like a continuous saw-edge with the small cusp of each tooth not showing. In fact, this palisade of sharpened teeth does practically form a continuous saw like cutting edge in each jaw.
There is only one row of teeth in each jaw and they are sharper than almost any shark teeth. When the piranha snaps these teeth together, the points in the upper row fit into the notches of the lower row, and the power of the jaw muscles is such that there is scarcely any living substance save the hardest ironwood that will not be clipped off.
Unlike the teeth of most predaceous fishes, the teeth of the piranha are not adapted to lacerate, crush, mangle, or seize and hold the prey. There are not several rows of teeth in each jaw as in many sharks. There is but one, and this is adapted only for clipping off small pieces of the prey, which in some instances is much larger than the fish itself, but is probably smaller most of the time.
The best mechanical imitation of piranha teeth and jaws, and a very good imitation indeed, is a bear-trap, but one with the teeth so sharpened on the edges, and the spring so strong, that they would clip off the bear's foot instead of merely holding it.
The mouth of the piranha is small. The size of the average piece bitten off would be that of a large olive or nutmeg. The pieces are swallowed whole, and rapidly, and the fish snaps many times.

Water Conditions and Filtration

The water temperatures in the piranha tank should lie within the range of 24 to 27C (75-81F). An average temperature of 26C (79F) is best. The thermostats available on the market control the operation of the heater and ensure that the water temperature is more or less constant.

Where the water chemistry is concerned, piranhas seem to be amazingly tolerant. Although it is generally recommended that they should be kept in soft, slightly acid water with a total hardness of 5-10 DH and a pH of 6.5, fish kept at the Zurich Zoo actually spawned in much harder water (18-19 DH) as well, according to Honegger. Higher pH values are tolerated, even those above neutral (7) and up to around 8.5. Soft and slightly acid water more like that found in tropical conditions is produced, for example, by filtering over peat or by adding either peat extracts or tannic acid in liquid form. Your local pet shop should have these. The filtration should be carried out by means of efficient outside filters. Where the water enters by a jet that has been fitted above the surface, any additional aeration is generally superfluous.

Light Conditions
Piranhas are by nature very shy and nervous, and they take fright extremely readily. If the tank is exposed to glaring light, the characteristics just mentioned noticeably intensify. It is therefore advisable to keep the fish in fairly soft light conditions. This can he achieved, for instance, by using warm white fluorescent tubes, by covering the aquarium with a pane of frosted glass, or by cultivating a dense cover of floating plants. Pet shops usually sell warm white tubes.