Koi fish need extremely clean water. Koi and pond plants are excellent for one another. Koi waste feeds plants and the plants create oxygen and their shade helps keep the water cool to protect the koi. The majority of fish keepers that have experimented with plants and koi do agree that koi at some point or another will test the plants to varying degrees from nibbling to all out ravaging the plants. Most ponders notice it's not so much the leaves of plants as it is the tender roots of plants the koi will seek out.There are many plants which have koi proof roots. Water iris, umbrella palm, sword plant, cattails, carex to name a few can withstand some of the worst koi grazing. The fish also seem to prefer young plants. Potting your plants in a solid container is also a trick that will keep those tender roots of plants safe and protected. The perforated black plastic pots the aquatic plants come in are of no use, as the shoots soon come out the holes and into exposed water.To keep the koi out of the plants, plant in heavy or large bottomed pots. You can use wire mesh or plastic grate on top to allow the new lily stems to come up, but won't allow the fish to root in your planting media.
Experienced keepers have found introducing young koi in a well planted pond often will familiarize the fish with the plants and help to keep the odds in favor of the fish not eating the plants.
If you have older fish, supplying them with a regular feeding of vegetable and greens will often take their attention away from plants. Koi enhance their own color by eating certain algae. The last thing you should want to do is wipe out algae populations with controls like copper sulphate. Instead, make selective use of products like Algae Fix, or natural controls like barley straw. These will reduce populations, leaving algae available for koi feeding.
In addition to helping control algae, koi also help control mosquitoes by feeding on their larvae, reducing the need for pesticides.