"Piranha breeding is half care and half luck, just enjoy and let it happen"
The first step in breeding piranhas is choosing the correct combination. I have found having an even mix of thick and thin piranhas is the first requirement. Thick piranhas are usually female and thin are normally male. For breeding your piranhas will need to be 4-6 inches, and they should well establish, being together more than six months. It is sometimes possible to find the correct combination of piranhas, size and gender, at your local dealer. Just having the correct combination does not guarantee success; you must set the breeding atmosphere. Your aquarium has to be large enough for the mating ritual and have the hiding places the breeding pair. (For more information on aquariums see the Aquarium set-up page.) I recommend for breeding an aquarium of 55gallons and larger, 100 gallons is ideal for six piranhas.
Once you have you tank set and you have a good mix of fish, the conditions must be set or mating will not take place. First the water temperature should be a little warmer than normal, about 82 degrees has been my lucky temp. Also you should test the water for correct pH and test for normal ammonia levels. Test kits can be found at most local pet stores or discount stores. Next, live feeders in the tank seem to help the process along. Try to keep a feeder in the aquarium at all times , you will notice that the mating pair will eat more before mating and almost not at all during .
By this time, if you are lucky to have a mating pair you will need to recognize then and observe the pair. The pair will separate from the rest of the group they will decide on a territory and defend it from intruders. They will continue to defend this area throughout the mating and while the eggs are present. There may be more than the usual amount of bites in the tails of other piranhas due to the pair. For about a day or so before the eggs are laid the pair will eat very little and stay in their chosen area. If your piranhas are easily startled try not to disturbed them at this point. After the mating , the male protects the eggs while the female goes back to her normal behavior.
Caring for the baby piranhas
Now that the eggs are there you must prepare to care for the the young to ensure their survival. First, set up a separate tank (5-15 gallons), use water from the original tank and be sure to use an under-gravel filter only instead of a power filter. Next watch your eggs and when you can see the fry swimming it’s time to remove the babies. Carefully siphon the eggs, take care because the male is still protecting and will attack, you may remove him before starting. Place the fry in your pre setup tank carefully. Finally you must feed your fry , the babies eat brine shrimp. I've had some success with the frozen brine shrimp but its better in my experience to produce live ones. Your local pet shop should be able to help you get started with the live brine shrimp.