What kind of fruits and veggies are good for my fish?

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I have 2 Shubunkin goldfish in my tank, and I read online that fruits and vegetables are good for them to eat in addition to their regular flake food. I feed them once every two days, no more than they can eat in three minutes, and I need a list of fruits and vegetables that are good for them, how large I should cut them, how often I should give them the fruits and vegetables, whether or not I should feed them the fruits and vegetables simultaneously, and whether or not I should in addition to the flake food, or separately. Thanks.