yep koi are carps, as al carps they are omnivorous, they eat everything
i feed mine the normal dried goldfish/carp pellets and on occasion lettuce, oranges, snails, fly maggots and earthworms

for their care : because of their size ( sometimes up to 3 feet) they need a very large pond and a very good filtration system
clean water is your first priority, this means absolutely no NH3,NH4,NO2,NO3 and and a stable ph level(6-8)

I used to have native australian silver perch called "junior mangles" after Alby mangles. He ate every other fish I tried to put in with him. I loved him heaps......he was a real character and would get the sh*ts if you moved anything in his tank and move it back with his mouth. People try and tell you they are stupid. Not true. R.I.P Junior

that's a very nice tank , you can find some really great aquascapes on the web, what you don't see in the picture is the filtration system, it's probably just as big as the tank itself