OK, goldfish don't have the same digestive systems like other fish!  CARBS!!!!!!! they require special food labeled GOLDFISH or KOI food!  high carbs and no fats!  potato flakes are awesom for them.  peas fill them up but offer little nutrician.  Hikari makes great pelleted foods for them.  seek them out!  they have floating and sinking types that will grow you happy fat goldfish and koi.  remember they like cool water and the tropicals don't.  maybe a second tank for them without a heater?  lots of water changes and DON"T OVER FEED!!!!!!!!!