Vanica, I'm having the same problem with my Sanchezi lol
He would always wolf them down immediately, but recently has left them alone. I also breed cons for live treats and threw about 6 in there; hasn't touched them. I guess its only a matter of time.
I guess my input would be give it time...the cons might need to grow some more in order for your piranhas to deem them worthy of their effort.