The Horn wort needs to float freely in the water like a forest of floating beautiful grass, and yes the koi will eat lot's of it in early spring. Horn wort is good food for all the fish, but in the early spring it can seem like almost all the horn wort is gone, but little bit will remain in pond and when the weather gets hot the Hornwort should outgrow what the fish can eat. When you get a snip of hornwort, put some in a jar in the window with water to grow, or in a well lit fish tank. Always have reserve hornwort just in case the fish were extra hungry and eat it all in the spring, until it gets well established in your pond.
In the summer, when the sun is very hot and high in the sky, that is the best time for hornwort to grow, as it usually will outgrow what the fish can eat. If you have lots of fish and only one piece of hornwort, then they may eat it all before it has a chance to grow, that is why you need to grow a snip of hornwort in a jar or outdoor pool with no fish. Hornwort likes lots of sun but will not do as well in pea green water as that blocks too much light. Once you have a good amount of hornwort established in the pond, it will come back each year and does very well over the winter in ice cold running pond water with the fish.
This was the hornwort in January..
This was last fall...
This was last summer when the hornwort was growing at its peat...