Today someone abandoned a common snapper in my front lawn. When I found it, they had placed a carrot slice on its back. Currently, he is living in an old aquarium half filled with water and it doesn't seem interested in the guppies that are in the tank. My main concern is that this turtle has some sort of nutrition problem from being fed just vegetables as it only moves to breathe or approach someone near the aquarium. He also didn't even hiss or attempt to bite when he was captured. Does it sound healthy and is it safe to release it where I know there are other snappers? Any help is welcome! Thanks!

P.S. I live in a state where selling turtles (but not buying them) is illegal.