Poor guy! It may be a case of dropsy. Google it and see if the symptoms and description fit your case.
Dropsy is usually associated with stress factors, fish aggression, or poor water quality. Goldfish seem to be very susceptible. If you think it is dropsy, you may want to check your water parameters and take steps to correct any problems you find. Other possible causes could be aggression from the tilapia or a recent stressor in the last week or so (i.e. a dramatic pH change or something similar).
(i.e. a dramatic pH change or something similar).