Regarding your feeder fish debate.
There are two ways feeder fish are unhealthy and bad for your piranhas.
Firstly most 'feeders' brought from bait shops and local fish shops are raised in unsatisfactory quarters to keep the price down. These unsatisfactory quarters are rife in diseases and infections within the feeder fish. Which of course get passed onto your piranhas when they feed on the feeders.
The second bad thing about feeders is 'carp like fish'. These include goldfish, koi, minnows etc. Such fish contain a vitamin which can restrict and slow natural growth of piranhas.
I suggest, if you are looking for healthy feeders, breed your own. While if you want to replicate their natural diet, try breeding or purchasing South American fish.
Also, earth worms and bloodworms are great for little piranhas!