I haven't heard anything about aquarium salt relieving stress. As Safdar said, it helps prevent fungus issues in the tank.
There is always going to be some stress on the turtles....heck, they are in captivity! But you can alleviate some of it by "teaching" them that you are not a threat. One of my fav tricks is to divide their daily portion food into, say, 5 pieces, then every time you go by the tank, you give them a piece of it till all 5 pieces are gone. That way they get used to associating you with food. This works, but you have to be patient. And it may not stop the biting when you pick the turtle up, but it can help alleviate some of the stress with the turtle seeing you always coming so close to him.








