And that can kill the fish. For proof, consider that a goldfish in an aquarium could live for 10 or more years, while most fishbowl goldfish would be lucky to make it to 10 months.
Gregory A. Lewbart, VMD, a professor of aquatic animal medicine at North Carolina State University, notes that one of his clients - who keeps her fish in a 1,000-gallon outdoor pond - has fish in their teens and a 30-year-old who just passed away.
"Remember, fish must do everything in the water where they live," he tells The Dodo. "Eat, urinate, defecate, mate, breathe. They can't escape their environment."
Which is why filtration is so important. Bowser recommends a fully equipped aquarium with a bio-filter - commonly placed under the gravel or on the side - that avoids the toxic combination of ammonia and nitrite. Lots of space helps, too, and Lewbart says an aquarium should be no fewer than 29 gallons total for two or three small fish - and a single 3-ounce fish should have at least 10 gallons, if not much more.