How to protect pond plants from overzealous turtles

This is my second year with 5 RES in my 8000 gallon koi pond. This year they are destroying my lilies. I'm happy for them to eat the greens but they are killing the lilies.
I have tried offering leafy greens but they are not interested. They take a bite and then spit it out.
Does anyone else have experience with protecting their plants? Also, what greens do you feed your RES? I saw a floating ball thing to put greens in, does anyone have experience with that?
Thanks for any advice you have.

Does anyone else have experience with protecting their plants? Also, what greens do you feed your RES? I saw a floating ball thing to put greens in, does anyone have experience with that?
Thanks for any advice you have.
