Every summer I eat lots of watermelon and so do my koi. Watermelon is a healthy fun snack for your koi during the summer. I usually eat almost down to the rind then toss the rind into the pond and the koi go crazy. After about 10-15 minutes they have eaten off the rest of the flesh and I net out the remaining rind. Here is a list of fruits and veggies that are great to give your fish as a treat.
Fruits: First off wash your fruits and remove the seeds. Some seeds can be poisonous or hard to digest.
Watermelon (do not feed too much this is why I just let mine have what is still on the rind)
Oranges/Lemons/Grapefruit (peeled, cut into halves or quarters great source of Vitamin C)
Cantaloupe /Honey Dew Melons (cut into 2-3″ cubes)
Vegetables: Wash before giving to fish
Zucchini (peel and slice long ways)
Broccoli (cut mainly like florets)
Use fruits and veggies as a supplement to their diet not a substitute. Koi prefer carbohydrate rich foods in the fall/spring and protein rich foods in the summer. Remove what they do not eat after 10-15minutes. Koi like any pet may or may not like these treats it may take some experimenting to figure out which treats they prefer if any.