If I add saltwater to my freshwater aquarium, will it kill my other freshwater fish?

I have 2 Spotted Puffers. I think they may be sick. They just lay on bottom all day, and turn black sometimes. I also have a black Mollie, 3 Tiger Barbs, 1 Algae Eater, and 1 Black Fin Shark. I read up on the puffers, and it said that they may need saltwater to survive. They are my favorite fish out of all of them, and I want to do everything I can to keep them alive.