As we feed them more and more, the waste products form more and more ammonia and nitrite, until the water becomes toxic and they die. What we are really doing is overfeeding our filtration system. I'm not sure we can ever overfeed Koi. The most important tool we have to know when we are feeding too much is an ammonia test kit. Remember, the feed turns into ammonia and phosphates.
Ammonia is converted to nitrites, which are then converted to nitrates.
Plants will eat up some of it, and water changes will help get rid of the rest. They start recognizing us, come over to get their food, will eat out of our hands, etc. Some people feed them spinach, bread, triscut crackers, whole peas, vitamins, watermelon rind, etc. If you want some more ideas ask some carp fishermen what they use as bait. Some have great secret formulas.