Ed Warner, in his book, "Success With Killifish" recommends using salt at a rate of four teaspoons per ten gallons with the adults, eggs and fry. This was a common practice among aquarists in the olden days of the 1960's when I started in the hobby. Last year I went back to doing this and my fish look better than ever. I haven't had a dead one in a long time and the water even has a luster I haven't seen in a long time. Those old timers like the late Mr. Warner are still giving good advice.