Swollen Abdomen could happen because of Bacterial or Viral infections, often carried in contaminated water or infected food, the fish comes in contact with.
There are various cures available according to the different stages of the fish with the Swollen Abdomen condition. Antibiotics are helpful in treating the early stages to help counter this condition.
It is possible, if you are feeding supplied food, that the fish is constipated or there may be some blockage in its Intestine. A cure here is Peas, which can help relieve constipation in fish but I am not sure how to administer this to a mandarin.
Another situation that can be mistaken for Swollen Abdomen can arise due to overeating. Therefore, if the fish starts refusing food as stated above, it reflects a loss of appetite and may be the case of Swollen Abdomen. If however, this does not happen then, the fish is likely to be healthy. The resolve here is not to give food to the fish for some time and allow its bloated abdomen to resume its original shape. If you are feeding the mandarin supplied foods, this is a possibility; if you are letting it eat copepods, this is unlikely.
