Certified GKH Gardening Expert
The fish will dine on your plants, that is just how they feed.
They are vegetative by nature.
You can try creating a shelf of soil and rocks at the edge of your pond.
This will help create a division for the plants and fish.
Make sure you are not putting excess food into the pond. The food will settle to the bottom and the fish will get to the food by eating through the plant roots.