More than 1,200 fish of 24 species were found dead on Wednesday (8 November) at a Japanese aquarium in Tokyo due to lack of oxygen supply. The incident wiped out nearly 94% of all the fish the major aquarium hosted.

The Sunshine Aquarium was closed to the public since the mass deaths but resumed the display on Thursday. Only 73 fish survived in the facility as 1,235 were killed in total from the aquarium's largest tank.

Dead fish were first discovered by a security guard. The officials quickly attempted to probe the cause but with no use.

Japan's Kyodo news agency reports that the aquarium turned off a cleaning device – causing a decline in oxygen supply – as part of the maintenance activities. New chemicals were added to the water to treat unhealthy fish recently. However, oxygen supply is said to be rerouted through another equipment.

With a capacity to hold 240 tonnes of water, the fish-tank was 2.4metres high and 9.6metres wide.

"When the tank was designed, it was supposed to have the ability to maintain the concentration of oxygen even if [the] oxygen supplying equipment was temporarily suspended," an official from the facility was quoted as saying. "However, the oxygen ran low probably because individual fish had grown bigger."

The aquarium is one of the popular destinations in Tokyo.