ive never tried cabbage when i had my goldfish but i did try lettuce and they didnt even come near it
however the 3 things that they loved most were broccoli, oranges (cut in big circles), and fresh peas
i fed them those 3 things for the first 3 months and they practically doubled in size
infact one of the comets i had was only 2in big when i bought him and right b4 he died he was over 7in (died from overcrowding and high amonia levels) and i had only had him for about 8 months
try out broccoli and oranges those seem to be their favorites
they may not touch the broccoli the first or 2nd day but after that they will eat it like crazy