Do you have a heater in your pond? I was nervous about leaving mine in an unheated pond this winter when my surface heater developed a ground fault. We're much warmer in the winter here, so I'm impressed with zone four overwintering. I've had goldfish survive in what seemed like solid blocks of ice, but I'm much more protective of my fancy koi.
I've never noticed any growth once I stop feeding them, but honestly I've never actually measured my koi beyond a rough estimation.
