There are a lot of people that will tell you not to put Japanese Koi and plants together. Pet stores and internet articles suggest that koi will destroy any plants you put in your pond. I have been keeping Japanese Koi in my water gardens for six years without any Koi destroying any of the plants. The plants provide shade and shelter for the koi when spooked, and add to the water clarity.
Japanese Koi are by far the most beautiful and desirable fish you can keep in an outdoor water garden. In Japan, breeding koi has occupied a position between hobby and art form for more than two centuries. To keep Japanese Koi in a water garden you have to understand that they are bottom feeders who are most at home rooting around, in and among the rubble found on the bottom of their natural environment. As a matter of fact, the purpose of the famous whiskers is to help them root around more efficiently in the rocks / rubble. Putting a layer a gravel in your pond will give the koi something to root in and will also add to the natural biological environment. Plants will also benefit from being directly in the gravel. I always remove plants from their pots, and rinse the roots before planting in a bed of gravel, which adds to the natural beauty of the water garden.
It is true that koi eat plants. It is also true that in the warm summer months they are none stop foragers. I describe them as eating machines. That is why it is important to keep plants that they eat without destroying. My two favorite are watercress and marsh betony planted in a shallow high water flow area. Watercress is an excellent plant food source for koi. Watercress contains significant amounts of iron, calcium and folic acid, in addition to vitamins A and C, and koi love it. It is best to plant on a shallow ledge with rocks around it to keep the koi from pulling up the roots. You can buy it in the produce section of your grocery store. Just stick it in some shallow water in gravel and it will grow. Trust me, you will need the koi to help you keep these invasive plants in check and may still have to thin late in the season. These plants are excellent filters which will help keep the water crystal clear and the koi happy.
Floating water hyacinth can result in the koi eating the roots. I usually plant in the shallow stream just below the waterfall. Their roots bury deep in the gravel to provide excellent filtration. They tend to grow taller and bloom less. Mine have grown 3 foot tall and extend 12 feet down the stream and seem to benefit from the splashing of the waterfall. This is another plant that will have to be dropped in the fall. Pickerel Rush is another plant that enjoys the splashing from a waterfall. I use them to hide the liner below the waterfall and they have bloomed constantly all season.