Oranges and other citrus are an excellent source of vitamin C for them. I have strung them here on yarn because they would push them into the waterlily leaves and damage them. If you don't have plants you can let them float around and they chase the slices around, giving them not only their vitamins but exercise and entertainment.
They love to intermingle among each other and are very curious too. If you watch long enough you can see they have a definite "buddy" system. Don't you think this would be a great cover for "Super Koi Comics"?
It's so easy to allow the world's problems to fade away while sitting by the pond.
More phish photos: