Serra is a very slow grow piranha...with proper care and depending on the species it can grow up to 6-7" within the first year and every 1" per year after that.
feed them every day for 3-4time aday when young. once they grow up to adult they can only eat once a day and every other days.
Here is a list of food you can feed them now.
blood worm
pallets
shrimp
krills
scallops
silver side
smelt
clams
fish fillet
beef heart