the bigger the koi, the more they destroy the plants. If your pond has nothing green growing, like string algae, then they tend to go after the plants even more. I keep only large grasses and sedges and hornwort in with the really big fat old koi. They come half out of the water to eat the grass and rip the aquatic mint too, but these plants are large and planted in very large baskets, which sit right at the water level, so it is not easy for them to get into baskets. They can also eat large amount of hornwort, but that plant is a quick growing weed and this time of the year it outgrows what they can eat. I also throw excess hornwort that grows too thick from the bottom pond, into the top pond for the koi as well.
