My turtles are Red-eared Sliders,with a carapace length of around 8", and they do eat fish, as well as a variety of vegetation. And to answer your question, koi are descendants of the European carp which was also imported to North America in the 1800's as a food fish, and we all know how that turned out! They destroyed the habitat of native species, which largely preferred clear water, by constantly rooting in the substrate for food. And of course, few of us actually EAT carp, so fishing pressure is virtually non-existant, letting them multiply at an alarming rate. The last thing you want to do is add to the already huge population of carp! They are NOT native, any more than the House Sparrow, Starling, and who knows how many of our insects. John