I was told to use hornwort in my pond because the koi won't mess with it. At one time, at winter's end, I had it, I swear, 2' deep in my 30" deep pond - so much I could have sold it.
Then the koi must have figured out that it's edible. I can't keep it any more. What the fish don't eat, they tear apart, and the little pieces were clogging up my filter.
I still want underwater plants, because they filter and aerate the water, and hornowort is the cheapest around here. Now I put the hornwort in floating mesh baskets made of plastic patio screen material, where the fish can't get to it. It's growing and doing fine!