I had turtles in my pond before, they are fine. They won't eat your fish, as long as you feed them enough they won't ruin your plants by eating them either. They will hibernate, though and like someone mentioned, if you don't have a closed garden area they will be off, so try to make sure it cannot leave. My turtles were fine pets to me, actually. You should be fine and really enjoy having a turtle in your garden.