A major Tokyo aquarium suffered a massive die-off inside its largest fish tank late Tuesday and early Wednesday, likely due to lack of oxygen.

Sunshine Aquarium resumed public display of the tank Thursday after suspending some operations the day before and announcing that a total of 1,235 fish, accounting for 94 percent of the fish in its large Sunshine Lagoon tank, had died.

The mass deaths occurred after the aquarium turned off a cleaning device for the tank to enhance the effectiveness of chemicals added to the water to treat unhealthy fish.

It continued to supply oxygen to the tank through another device and had spotted nothing abnormal by Tuesday evening. The next morning, a security guard discovered the dead fish, it said.

Only 73 fish survived, which had 23 different species, according to the operator.

Source: ANNnewsCH