1. Water quality inspection: firstly, check whether the water quality meets the life requirements of goldfish. Ensure that the water temperature is appropriate, the pH value is balanced, and the amount of ammonia, nitrate and nitrate is appropriate. Sometimes, poor water quality can lead to fish health problems.
2. Isolation: If you find that fish only stay near the filter, and other fish don't have the same symptoms, consider isolating them in another sink. This can prevent the disease from spreading to other fish.
3. See a doctor: If the red plaque on the fish's stomach is not the normal color, or the fish's behavior is abnormal, it is recommended to consult a veterinarian or a professional aquarium expert. They can give an accurate diagnosis and provide treatment advice or drugs.
I suggest you consult a professional to get the best advice and treatment plan for your goldfish.