Looks like your goldfishes have a bad case of Dropsy. It's most likely a bacterial dropsy infection, and while there are many 'experimentataive' ways of treating this disease, none I have tried have been successful over the long term. Some ppl suggest adding epsom salt (ENO) to the water. There are propriety meds for internal bacterial problems u can try, but the best chance seems to be daily injection of an antibiotic like amikacin. This would have to be obtained from a vet, and very few vets would treat a fish. If u've exausted all avenues, it would be humane to put the fish down. I use an overdose of clove oil (can get from the pharmacy).

The underlying problem to dropsy is most likely hygiene. Is your pond running on a filter? Has the pond been cycled properly and what are the ammonia and nitrite readings? These can contribute to diseases in the fish.