true but we're here to share experiences and through our collection of experiences we should know by now that 6 in a 65 is not good advice.
piranhas can always turn on each other at any point regardless of tank size, size differences and feeding schedules but there's no point of forcing a territorial dispute by putting them in a confined space at their most cannibalistic stage.
most experiences also say that 3 red belly piranhas together are rather boring and not very interactive but larger shoals are much more active. My advice to everyone is to get the largest tank you can and aim for 20 per gallon unless you're looking at tanks larger then 125, then i'd consider overstocking