RES do like eating fish fry. This last winter I passively feed trained 30 small RES in my aquarium and they ate every single albino bristlenose pleco that hatched in the tank. I have one male pleco and four females, the poor male spent 5 solid months tending eggs and fry only to see them disappear once they left his spawning cavity. About every 5 or 6 weeks there would be a new batch of fry and I could always tell when they had hatched as there would be one or two RES intensely focused on the spawning cavity waiting for fry to swim out. The RES would aggressively chase other RES away from the cavity while waiting for fry to emerge. This aggressive defending of a "feeding spot" would go on for about 5 days until all of the fry were gone. Normally I see very little aggressive behavior with the RES in my aquarium, but when I do see it, it's very short lived, the only exception I saw to this was when fry were present.