Goldfish do not have a stomach. The function of the stomach such as digestion and absorption of nutrients is carried out by different parts of the digestive tract.
Goldfish can thus keep eating continually. This can generate a lot of waste which can overload the filtration system.
Goldfish need to be fed small amounts 3 or 4 times a day. Since Goldfish do not have powerful digestive capability in the absence of the stomach, Goldfish should be fed only soft foods rich in fibre. Goldfish are very susceptible to constipation.