First, you will need a large tank for at least 5 adult piranhas 6 to 8 inches in length that have been together for a few months. I would recommend at least a 125gallon tank (if you want them all to survive through the spawning). Choose thick and thin piranhas of the same length, thicker ones are females and thinner ones are males. This method of finding males and females is not entirely accurate, but it is the closest you can get to find who the males and females are other than seeing who lays the eggs and who fertilizes them.
Breeding piranha is more pure luck than the "right" conditions. However, more frequent and bigger water changes seem to have a big effect on the success of breeding piranhas. The rainy season in the amazon is the time when most fish spawn. The more frequent water changes simulate the rainy season, probably getting the piranhas into breeding condition.
Breeding piranhas can be hard, they do it when they want to. When I bred my piranhas I didn't have any "special" conditions for them, I just kept them like I always did. If you follow my method of keeping piranhas, then they will probably spawn when they are ready to.
One of the signs of piranha in breeding condition is that they lose all their color and turn almost completely black. You may notice 2 of them starting to guard a certain spot in the tank, while chasing off other piranha that come to close. If you observe 2 of them picking up gravel in their mouth like they where digging for something, then they have already begun the mating process (its best not to disturb your piranhas during this time). They will dig a pit as deep as they can and may line it with small clippings from live plants (if any are in the tank). Then the male and female will hover over the nest and align their bodies side by side and start rubbing their anal fins together, this ritual can last for hours before any eggs or sperm is released. The female then releases eggs and the male releases the sperm to fertilize them. The eggs then fall into the pit, the female may leave the nest or stay close by, but the male is usually the one that takes responsibility of guarding the nest and eggs. Piranha eggs hatch in 2-3 days, so be ready for them!
If your lucky enough to get this far, then its time to take care of the fry. Once the fry hatch move them into their own tank, with water from the parents tank. You must be very carefull when doing this though, piranhas become VERY aggressive and protective of the fry & eggs during this time.
Keep the fry in a bare tank, with only a sponge filter. The fry quickly absorb their yolk sac and you will need to have food ready for them! You will have to feed them newly hatched brine shrimp. Ask your fish store about the brine shrimp eggs and how to properly hatch them.
