Lee's comments are spot on as per usual. We treat ours much the same (although I haven't tried bananas yet...) Very good advice.
To add to the list a bit, the only lettuce that has any nutritional value is the darker types of leaf lettuce. Iceberg (common head lettuce) has none. Spinach is a very good choice if your Koi like it.
Many ponders also feed cooked pumpkin with good results. Our fruit and veggie treats really tend to be fresh ones left over from our own table. (no corn on the cob though

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