The main reason is that the aquarium water is too old. Generally, these fish will have symptoms when they are sick: tiny yellow powder will appear on their bodies and fins, just like peppers, and the fish will cling to all the fins and scurry around in the water. In fact, this disease is usually caused by lazy fish friends, and the water in the aquarium has not been changed. If the sick fish are not treated in time, they will generally die soon.