The hoarfrost on koi fish is caused by water mold. This is a disease that is harmful to many kinds of fish. There are many pathogenic bacteria, usually water mold or cotton mold. When a fish is injured, germs will invade the body from the wound and give birth to many hyphae to wrap the fish, making it look like a layer of frost. Attention should be paid to prevention at ordinary times. Because this disease is often suffered from at low temperature, it is very important to raise the water temperature.

1. Change water: The first thing to do to treat water mold is to change water. The purpose of changing water is to reduce the content of fungi in water and improve the water quality, which is very helpful for subsequent rehabilitation. It is different to change water in peacetime. During illness, it is basically necessary to change water every other day, about one fifth of each time.

2. Heat up: After changing the water, the next step is to heat up, which can accelerate the metabolism of fish and improve its resistance. However, heating up also has certain disadvantages. As soon as the temperature rises, the reproduction speed of fungi will also accelerate, so it needs to be salted.

3. Salting: Salting can change the osmotic pressure of water body, which can reduce the infiltration of water into fish, reduce the burden on fish kidneys, and prevent some fungi from invading. The concentration of salt must be controlled, which is generally three to five thousandths, and it will not work if it is higher.

4. Medicated bath: If the water mold is serious, it can't be cured by changing water, heating up and adding salt, so medicated bath must be carried out. Commonly used medicated baths include malachite green, methyl blue, yellow powder, etc. Different drugs should choose different concentrations. Once the dosage is too large, it will also cause drug damage.