1. Disinfection treatment: Before the fish go into the pond, soak them in 3%-4% salt water for disinfection to help kill the germs on the fish surface.

2. Density control: When raising crucian carp, the stocking density should not be too high to prevent diseases of crucian carp due to overcrowding.

For the case of blood on the scales and mouth of crucian carp, it may be that the crucian carp has a hemorrhagic disease. This requires effective preventive measures, including:

1. Disinfection treatment: Before the fish go into the pond, use quicklime to clear the pond for disinfection, and then use water body for disinfection to eliminate pathogens.

2. Control water quality: maintain good water quality and replace fresh water regularly to reduce the occurrence of diseases.

3. Feeding feed: to ensure the balanced feed nutrition, it is necessary to mix different feeds in different periods, which can ensure the comprehensive nutrition of crucian carp, and feeding a small amount of feed for many times can effectively reduce the occurrence of fish diseases.

If the problem has not been solved, it is recommended to seek the help of professional aquaculture personnel.