The city’s newspapers have also been demanding answers, though heavy state censorship prevents them from investigating bigger scandals. But the vanishing fish have become a powerful story, and a morality tale of the enforced ignorance and uncertainty that frustrates Chinese citizens.

The front page of The Beijing News newspaper on Thursday was dominated by a picture of the empty tanks at a supermarket.

“When the live fish mysteriously disappear from some supermarkets, when there’s a plethora of public explanations, when rumors and doubts abound, in the end it all comes down to having no sense of psychological security,” the paper said in a commentary published online on Thursday.

Not all supermarkets have removed their live freshwater fish. Visits to supermarkets across Beijing on Thursday found some were still selling carp, northern snakehead and other freshwater fish, which many Chinese prefer to saltwater fish. They often braise or steam the fish; chopsticks make it easy to deal with small bones.

But in many other stores in Beijing, freshwater fish are gone with no explanation, especially in recent days. At a supermarket in Sihui, a neighborhood in the city’s east, decorative gold fish were swimming in one tank that sales assistants said had been brimming with freshwater fish until a few days ago.

“We’ve been to a couple small and big supermarkets in the neighborhood, but there are no fish,” said Han Yi, a 24-year-old technology professional who was shopping with her boyfriend.

“Before, I always bought from the big supermarkets, so I thought they would be safe,” Ms. Han said. “Now I’m not so sure.”