imo really oily fish shouldn't be fed consistently but that dosen't mean you can't use it but the main part of their diet should be white fleshed fish (tilapia and catfish are great choices) and then a crustacean of some sort (shrimp or something like that) and then maybe a type of mollusk (squid cuttlefish, or octopus)
i feed mine catfish,mackeral (once a week only becuase it's oily be sure to do a water change soon after to remove any oils in the water), shrimp, octopus and hikari massivore pellets
oily fish can be part of a piranhas diet but really shouldn't make up more than 15-20% of it and you may want to do more water changes if your feeding them sometihng really oily i feed mine mackeral becuase i can get it whole (organs and all) so it's more nuitritious to the piranhas than fillets and it's dirt cheap ($3 for 8, 10"-14" fish)
maybe give the piranhas beef heart or chicken heart as a treat once or twice a month