The drop in salinity shouldn't hurt the fish. They can take it. The corals and other invertebrates are another thing. For the fish, I would suggest raising the salinity slowly (days). I'm not sure what to recommend for the inverts but slower is probably better than fast. You should be able to make most of your corrections with water changes. Have you tested alk, ca, mg? Whatever it is you can still correct with water changes. You can slowly bring up salinity by topping off with salt water instead of fresh water.