You can have koi and plants, you just have to prepare for what will happen, Koi will dig, to avoid this place lava rock or 3-4" pebbles on top to keep them from digging, marginal plants do better than lilys. and tender plants. Taro's, Iris, pickerel rush, thalia's, Papyrus all do very well with Koi, Small koi aren't too bad, I have kept lilys, and lotus with them but as the koi get larger they like to eat the new shoots, I've found that others are using netting with floating rings and it seems to keep the bigger koi out. So far I haven't had to do that but I've seen Big Koi and lily's so I'm still going to keep trying as long as I can get away with it. the one important thing though is give them something they can eat, I use water lettuce, I can grow some in one container and as it needs thinning out put it in the koi area, they love eating the roots, they love azzola and duck weed, If you have a skimmer these are a little of a pain to keep out but if you have a calm area and they are eating well they will eat most before it gets there. Mine also love canned green beans, baby lesuear english peas, oranges, limes and other fruits with seeds removed. They are pigs! I wonder sometimes if giving them these help fill their need for the plants???? Not sure on that one.