Also question is, what would happen if you swallowed a goldfish?
Short version: It'll die. Long version: Even the swallowing itself might injure or kill the fish. Even if the fish is swallowed with a lot of water, water can't dissolve much oxygen at 37 °C. So even if the stomach was filled with plain water, the fish will suffocate rather quickly.
Furthermore, what happens if goldfish eat poop? If they pick up something inedible or unpalatable, such as small stones or feces, they'll just spit it out and go back to swimming or searching for food.
Likewise, how do fish die when swallowed?
Even in the case of fish being swallowed alive, the high-acid/low-oxygen content of the stomach acid and chime present in the predator's digestive tract would likewise cause it to perish from suffocation fairly quickly.
Why goldfish die so easily?
The fish can get ammonia poisoning from the still water, their own waste, and the stress of being so active in an undersized tank. The stress and ammonia poisoning will limit their lung capacity and they'll pass away.
