My koi fish's stomach is swollen, and I don't know what's wrong with it. Is there any good solution? Thank you.

Generally speaking, enteritis can cause the abdomen of fish to enlarge, and so can some parasitic diseases. You can take it out and pinch it to see if there is any abnormal color liquid flowing out of the anus, see if the anus is red, and observe the state of eating fish. If these are not good, you can add some enteritis spirit appropriately, stop feeding during taking the medicine, and then feed after returning to normal.