RES eat pellets well just not at the surface. I have many eating as the pellets sink down about 6-8 inches.
I have RES in an Intex swimming pool that eat Optimal BG fish food at the surface just like the BG in the tank next to them.
I DID lose about 150 RES out of 1,000 that wouldn't eat the food. They were approximately 3" long when put in the swimming pool.