Koi and plants do fine if the plants are abundant enough. If you “try” one or two plants to see how it goes the group of Koi will consider them quite the novelty and will probably allocate a lot of time to checking them out. Not having thumbs, they experience their world with their mouths.
If there’s ANY gravel in the pond, they will spend time sifting that, and rolling the rocks around, and playing with the plants, and then the plants aren’t the sole entertainment.
