The red flag fish has a layer of frost on its dorsal fin and a large piece of its tail is missing. It seems to have been bitten by another fish. I just got back less than a week ago. . What to do.

In that case, your red arrow wound is infected by germs. The first step is to raise the water temperature to 30 degrees. Second, buy some Japanese yellow powder or gentamicin, and sprinkle some salt if it is not available. Your fish will be healthy in a week. Adjust the temperature back to 26 degrees when you are healthy. Change a quarter of the water every week.