This is a bacterial disease you are talking about. You get all the fish out. There are bacteria in your tank. This bad disease needs time to be cured. First, isolate your fish. If one fish is sick, do not spray the entire fish tank with medicine. Go to the fish pharmacy and buy some medicine. Buy sterilizing water. Use some every day. On the bottle. Will tell you how to use it. Also, pay attention to water quality and ensure the water is clean. Seven-star saury is a purifying water fish. You can also put Qingda in your fish tank. . However, there is a ratio of 90 liters of water and 2 bottles of Qingda are enough. That's the ratio. If nothing unexpected happens, it will be fine in less than a month.