sorry to hear that the last one had died as well. never had piranhas die like that, if they were healthy from the beginning they shouldn't die off like that if your tank is properly cycled.
you said you had cichlids in there before you get the reds, how long was the tank empty (has water but no fish) between the cichlids and piranhas?
do you treat your tap water when you do wc? ph of your tank, temperature?
were they eating at all since you got them?