Damian, I can see why you want to keep rbp's. They are magnificent. And the fact that you came here seeking advice indicates your interest in their well-being. The best advice I can give you is to take some time to read, read, read, anything you can find online in regard to rbp care. Googling red belly piranha care is a simple and productive first step. Pay attention to info specific to the species that you own.
As you read, evaluate your tank size and setup, the number of fish you are keeping in regard to their natural behavior alone and in groups of various sizes, your feeding plan, your water conditions. As you read, many of your questions and concerns will be answered, including why this terrible occurrence took place in your tank.
Don't stop at one article, or three articles, or six. Keep reading until you have a good overview of rbp care and you aren't coming across radical new ideas anymore --- in other words, you already pretty much know what the articles contain. Today you can begin to get a pretty good idea about rbp care, but keep on reading for a while. Education is your best defense against fish loss and also against bad advice. You yourself should be aware of what is in the best interests of your fish.
Also, once you've educated yourself, you'll have better interactions with the forum members here, as we've all had to do the same thing, and none of us are equipped to take the place of all the information provided online on caring for and housing your fish.
In the meantime, as you read, just keep the water very clean in your isolation tank, along with the other measures set forth above that have been working to restore your fish's health.
Good luck with the information-gathering, and please keep us posted as to your fish's recovery and any recovery-related issues.