What sort of fish? My loaches & plecs eat romaine, leaf lettuces, zucchini, peas regularly but sometimes others like broccoli (cooked stems, the flowers are messy & stinky), summer & winter squash, bok choy, brussel sprouts, etc. Spinach, carrot & apple are not favorites, banana is messy.
Barbs, SAEs, garra like veggies too. My apistos, corys & rasboras do not. I put veggies in homemade discus food too.