Causes and solutions to the bloated belly of koi fish:

1. The bloated stomach of koi fish may be because it eats too much. If the breeder gives too much feed, the koi will not know when it is full and will keep eating causing bloating. Breeders need to stop feeding once or twice to reduce the burden on the koi's stomach.

2. Under normal circumstances, it is normal for koi fish to gain weight after being raised well, which means that the fish is in good condition.

3. It may be because the koi is pregnant. When a female koi becomes pregnant, the eggs in her belly will develop. It can cause bloating and, to some extent, koi production.

4. It may be because there are parasites in the koi’s stomach. Parasitic parasites can cause water to accumulate in the koi's stomach, causing swelling. Breeders need to treat diseases promptly and use repellents to repel insects.

5. Generally, enteritis is caused by unclean water or overfeeding. Koi enteritis is also called rotten intestinal plague, which is caused by Aeromonas punctata enterica. In severe cases, the mortality rate is 100%. Ninety percent of the time, it is best to change the water in time and feed them appropriately.

6. It may be that the koi is suffering from ascites. You need to change the water in the fish tank first, then keep the temperature at about 30°C, and add appropriate salt. If it does not improve, use yellow powder or malachite green medicinal bath.

7. You may suffer from intestinal phlegm. Intestinal phlegm is a multiple disease. Overfeeding at low water temperatures will cause this situation. It is best to fast for two days and observe. Then soak the fish food in garlic juice, dry it and then feed it for a while for observation.

8. If the cause cannot be found, the koi may have other diseases, and you can ask a professional veterinarian to treat it.