According to Dr. Brown, fish diet researcher at Purdue it's excess carbs in the artificial diet that are creating the excess fat. That said some types of fats in feed don't dissolve well in cold water digestive tracts.
I has a 7 1/2 inch YOY yellow perch die in one of my tanks today. The netting I use to keep them from jumping out had come loose where it's fastened to the tank and the perch had become gill netted in it. Anyway I skinned it out and will mounting it in a pike's mouth. It's abdominal cavity was loaded with fat.
