Step 1: Is it safe to keep piranhas?
Piranhas are only dangerous in large tanks or shoals.
In smaller tanks of 3-4 fish they are not overly aggressive to humans.
Red-bellied piranhas are best for beginners.
Yellow-bellied and black-bellied piranhas are more aggressive and more difficult to keep.
Step 2: What do pet piranhas eat?
It is illegal to feed piranhas live food, or buy live food such as goldfish to feed to piranhas.
There are plenty of alternatives such as frozen mussels, cockles, lance fish. etc. which piranhas will eat readily.
Do not overfeed piranhas. Excess food will remain in the tank and rot. If there is leftover food in the water after feeding, remove it with a net.
Step 3: What special requirements do piranhas have?
A piranha tank needs extra filtration - double the amount as for other fish. Use one internal and one external filter to keep water extra clean for them.
Tank should be well planted with lots of cover. Piranhas don't like bright tanks; they like to have lots of hiding places.
Good luck keeping your piranhas. They make wonderful pets.