Hi if your fish was storing eggs it wouldn't be below its gill area the fish would be rounder near its abdomen/back end is there any chance of a photo so that we could see maybe what the swelling is and hopefully be able help
Sorry chemblue been busy so not had chance to reply so hoped someone else would in the meantime anyway looking at the photo the goldfish does appear to be more rounder on one side, and as breeding is in spring it is likely that it is a female with eggs kind of difficult to see clearly on a pond photo but there are no scales standing out are there I had a common goldfish that swelled up and it was a suspected tumour but yours doesn't look quite the same so hopefully it is breeding and nothing to worry about please let us know how your fish gets on and if you see any fry
Do I need to do anything or just wait for nature to take it's course.
Basicly yes although have you seen any chasing going on, this is usually a sign of breeding although males will chase males too does it have any plants at all because this will give the eggs something to cling to and somewhere for any fry to take refuge, otherwise they will be eaten by the adult fish can you tell us a little about your pond, what fish are in it, how big etc any questions just fire away ok
Do you remember me, I sent a photograph and asked if my goldfish was pregnant 22nd March 2012 - the swelling has not gone down and she now appears to be wanting to swim on the surface and poking her nose into the air.
Look forward to hearing from you or anyone who could assist.