How much salt do i use for my red eared turtles?

I have 2 red eared turtles and i think they have eye infections, Their eyes are white and puffy. They are little, about an inch and a half wide. I have heard that you can put them in some salt water to help it get better, But how much salt should i use if i am puting them in a little tub with 3 cups of water? Thanks!