Go to your local pet store and buy some rosy red feeder fish, 50-75 should work I started with 25 and now I have a couple hundred. Just throw them in the pond and they can last over the winter as long as it's deep enough
My rosy reds had no special environment, just a big enough pond so that the koi didn't eat them all right away, not sure how quickly they breed but I can tell you that I threw 25 adults of them in my pond towards fall of last year, when spring time came there was at least 100 babies swimming with the adults, as those ones grew to max size quickly many more babies of different sizes filled the pond, now I have a huge school of feeders at a variety of sizes of different generations, best of all they just eat excess algae in the pond and koi food scraps. Hope this helps!