Well get them on pellets ,, much cheaper then goldfish and wayy more nutritious ..
You could get some frozen shrimp,tilapia,salmon,catfish ,cut in chunks ,add some vitachem and freeze it.. i use it together with pellets and blood worms (mine are 3-4inches) and they have awesome color and healthy
Don't feed goldfish at all as they are members of Cyprinid family (carp fish) and should be avoided, as they have growth-inhibiting hormones ( Thiaminase/Vitamine B1 inhibitors), which negatively affect your piranhas health and development...i would feed them minnows or tilapia or any other type of fish...fresh fish section at your grocery shop has a good selection...and feed as much as they can eat in 3-6 minutes 4 times a week...
I don't like goldfish very recyclable fish and all sick although they don't give anything good from vitamin.
I personally have 2 adults like poop machines and feeding them 3 times per day frozen shrimps...
It cost me like 12 euros per month.
You can try shrimps, frozen mouse, fresh fish.