My experience was my RES ate every single plant I tried. Water
Lilies, floaters, duckweed, frogbit, hydrocotle, Vallisneria, hygro, guppy grass, etc. he had plenty to eat in terms of guppies, and floating pellets, but always chowed down on the plants. My local turtle club also expressed the same experience. RES will eat plants. YMMV but the odds are against you.