I know it isnt the best idea to feed piranhas store-bought feeders, but besides the point what is the method to quarantine them? I have a 30 gal feeder tank. How long should they be kept in there after i buy them? Should i put certain medications in?
I would keep them in the 30 gallon tank for no less than 3 weeks. You don't need to medicate them unless they are showing symptoms of illness/disease. Yes, as said, gutload them with all kinds of healthy foods. This way the piranhas will benefit from the nutrients in the feeder fishs' stomaches when they are eaten.
I personally think it is good for piranhas to have a healthy, quarantined, live snack once in a while.