Answer

A distended abdomen is usually a symptom of a disease called Dropsy but as Dropsy causes death within a few weeks it's safe to say the fish does not have Dropsy as it has had this problem for about a year. Dropsy is a fatal disease that affects a fish's kidney making them unable to expel water from their body thus the swelling.

 

When Dropsy is not the cause then the next culrpit is something called being

egg-bound which is essentially a female fish being unable to expel her eggs.

 

Being egg-bound is not uncommon in goldfish and some female goldfish can actually live a long time with it. Egg-binding becomes a problem when the eggs that are internally stored inside the female begin to rot and become infected.

 

What I recommed be done is isolate the affected fish. In their isolation tank/pond add some epsom salts at a ratio of 1 teaspoon per every 5 gallons/20 litres of water.

Epsom salt is sometimes effective in coaxing a female's body to expel the eggs.

 

In addition to the epsom salt begin to treat with an anti-bacterial medicine such as Maracyn-Two. The purpose of the medicine is to prevent or treat any bacterial infection that might be occuring in the eggs. Maracyn-Two can be easily found at most major pet stores. Just follow the directions on the product label for usage instructions.

Note: if you live outside of the USA please let me know and I'll recommend a medicine easier to find.

 

Keep the affected fish isolated for 14 days including and after the medicine treatment has finished.

 

If you encounter a problem and are unable to isolate the affected fish you could use the same recommended treatment in the main pond but a large (50%) water change would be needed once the treatment has finished in order to lessen the strength and potency of the epsom salts. It would also be better, if treating the main pond, to use a pond oriented medicine such as TetraPond Pond Fish Treatment as it's more economical. TetraPond can also be found at most major pet stores both in the USA and outside of the USA.

 

Best wishes and please let me know if you have any questions.

 

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