Causes of goldfish’s big belly:

1. Overfeeding. Goldfish can be overfed, so don't feed them frequently, but feed them every day or two.

2. Breeding period. The goldfish's abdomen becomes enlarged, or it may be during the breeding period. The abdomen of goldfish during the breeding period is usually bulging.

3. Ascites. This disease is a disorder of body fluid circulation and metabolism in goldfish caused by bacterial infection. Specific symptoms include swelling of the goldfish's body and protruding eyes.

4. Swim bladder loss. When it is found that the belly of a goldfish is facing up or down and its swimming status is abnormal, it may be due to the loss of its swim bladder.

5. Bacterial enteritis. At this time, the goldfish's abdomen will become enlarged and red, the anus will protrude, the spirit and appetite will be poor, and the goldfish will twitch and pass white feces for a short period of time.