Sliced orange, cucumber/zuchini, peas (not a good staple food, but they'll help relieve constipation/bloating), or anything leafy and green (spinach, leaf lettuce, romaine lettuce, etc.). Those are the one's I've heard suggested.
i give all my fish (except the chiclids) fresh bits of romaine lettuce hooked onto an all clear glass christmas ornament ( put it upside down so it floats and use the hook to attach the food) And all my fish even my pleco swim up and nibble on it. from my experiences goldfish will eat anything but it might not be very healthy for them lol.