Photo/IllutrationSunshine Lagoon, a water tank at Tokyo's Sunshine Aquarium, in which a mass death of fish occurred. (Asahi Shimbun file photo)

More than 1,000 fish of 24 species died in their tank at Tokyo’s Sunshine Aquarium because of a mistakenly reduced oxygen supply, an aquarium official announced Nov. 8.

A keeper at the popular tourist attraction in Toshima Ward discovered 1,235 marine creatures kept in the large Sunshine Lagoon tank had died early that morning.

Eagle rays and double-lined fusiliers were among the dead. Of the surviving sea creatures, the facility resumed exhibiting zebra sharks and about 70 other specimens from the morning of Nov. 9. It intends to gradually reintroduce the number of creatures on display.

Medicine had been put into the water tank on the morning of Nov. 7 to treat some of the creatures that were suffering from disease.

Staff removed protein from the tank and temporarily stopped the machine that supplies its oxygen, aiming to increase the medicine’s efficacy. Oxygen continued to be delivered through other equipment, but apparently in quantities insufficient to keep all the fish alive.

“When the tank was designed, it was supposed to have the ability to maintain the concentration of oxygen even if oxygen supplying equipment was temporarily suspended,” the official said. “However, the oxygen ran low probably because individual fish had grown bigger.”