Ya I agree with you on the coloring, but I dont think it looks anything like an altuvie. If you look at the pics of their xingu on aquascape it looks exactly like them. Plus it says they are sold out of altuvie so I dont know. I am not saying your wrong as you have seen tons of more piranhas than I ever have. I just dont think it has the same head shape, jaw line or anything like an altuvie. I might put a pick of it up in the serra forum on p fury to get it identified for sure.
Thanks
E