You are right it is carp, I wonder where it came from, the pond where we found these fish has Koi & Goldfish only. No one from the outside has access to this pond and now we have hundreds of this little guys.
if that is a hint of green along the back it could be the grass carp that feeds on plant matter primarily and they are suppose to be good eating. that is according to the Asians.
Based on finage my bets still on regular goldfish just a dark morph.