This is not the first study to suggest that water-dwelling creatures can become aggressive in small tanks. Biologists at the University of Alabama, Birmingham, recently found that sea urchins turned to cannibalism when kept in small, overcrowded tanks. The researchers were trying to recreate typical farming conditions for sea urchin, a delicacy in Japanese cuisine.

Nor is the idea news to Justin Muir, the owner of City Aquarium, a luxury fish tank business in Brooklyn.

“It just goes back to behavioral exercises,” said Mr. Muir, who has designed tanks for the Yankees pitcher C. C. Sabathia and the Dream Hotel in Manhattan. “More volume of water is always the better bet. It basically keeps the fish healthier, and the tank is more stable.” In that way, he said, fish are like any other pet kept in a small enclosure. But a major difference between fish and, say, Rottweilers is that aggressive behavior in small swimmy things can be entertaining, at least to humans.

“That’s why these fish sell,” Mr. Muir said, “because people like the way they act.”

Indeed, hobbyists who probably don’t view themselves as diabolical gleefully exchange online tales of clashing cichlids. Describing a case of “road rage” between two of her fish, a visitor to Fishchannel.com wrote:

“The two would stand in front of the other twitching their lower fins as in sign language, yelling at the other with ‘You almost hit me you blind fool. Didn’t you see me coming? I had the right of way!’ ”

Overcrowding has also become an issue on fish farms, where salmon or trout are sometimes packed into high-density pens, just as chickens or pigs are on industrial farms. The danger there is less about the happiness of the fish than about their health, said Alan Duckworth, a research scientist with the Blue Ocean Institute in Cold Spring Harbor on Long Island.

“Some species do better at high densities,” he said, “but the majority of species could be affected by overcrowding. It likely would stress them out, which could increase the amount of disease.”

One frequently cited problem with farmed fish, particularly salmon, is sea lice, whose spread has become such a problem in Scotland that the government is considering a ban on coastal fish farms. For Dr. Oldfield, the welfare of fish is a concern that dates to when he was 6 years old and won a goldfish in a small bowl at a county fair. He says he understands that a 100-gallon tank is beyond the means of the typical tropical fish hobbyist, but people who love their fish should be aware of the damage they may be doing by keeping them in small, bland environments.