Common feeder goldfish, also known as comet goldfish, are usually a poor choice of feeder fish. They are very high in saturated fats. If you wanted to use feeder fish I would recommend livebearers, like guppies or Gambusia, as they have lower saturated fat levels. If at all possible I would set up a quarantine tank for any feeders you purchase to help reduce the chance of parasites. With guppies this is especially useful as they breed quite readily.
Also, you probably would not want to feed only fish to ensure that there are not any nutritional deficiencies.