If you look closely at your aquarium fish, the abrupt emergence of a conspicuously large abdomen can mean a couple of different things. In females it can indicate breeding on the horizon. In fish of both genders it can signify a health condition known as "dropsy."
Dropsy isn't a disease per se, but rather a symptom of a larger problem. It is the collection of fluid inside a fish's body. This triggers visible bodily swelling, frequently in the abdominal region. Many conditions can lead to dropsy, including serious cases of internal parasites, frustration due to bullying from other fish and extended time spent in water of unsuitable quality. The swelling is often accompanied by the body's scales jutting out prominently. You can often easily identify dropsy in a fish by observing him from above: he may have a pinecone-like appearance.
Prominent Change in Activity
An overly big stomach in a fish can bring on difficulties in swimming and floating in general. You might notice your fish descending to the bottom of the aquarium, or perhaps remaining at the top more than usual.
If your fish suddenly develops a big stomach and it turns out to be dropsy, the odds of his passing away are unfortunately high. Some fish with dropsy die promptly, while it takes others a few months.
Nipping Dropsy in the Bud
When it comes to helping your fish get through dropsy and whatever issue might be causing it in the first place, the sooner you act the better. Once your pet's belly looks massive and swollen, things might already be in a late phase. Some fish experience signs of dropsy before any swelling, including unusually antisocial behavior and absence of appetite. If you see these signs in your pet, talk to a veterinarian who specializes in fish. She might be able to instruct you on how to take care of the problem—and therefore eliminate the dropsy. Solutions could entail anything from antibiotics to simply improving the condition of the water in the tank—sometimes both at the same time.
Not Dropsy, But Reproduction
If your fish is female, a suddenly swollen stomach can mean that she's about to lay eggs or birth live young (if she's a livebearer). It's generally possible to discern between an "expectant" fish and a fish with dropsy: in stark contrast to fish with dropsy, expectant mother fish typically go about their business as always—showing no unusual behavioral shifts.