How to save a stressed Saltwater fish?

I got 3 brand new fish for my tank ( flame hawkfish, flame angelfish, orange stripped goby). 1 of my 2 original fishes, a lawnmower blenny was attacking all three. I have now traded him. But the orange stripped goby has died and my flame hawkfish is lying on his side breathing fast. How can i help him. I am thinking it was from all the stress.