TOKYO — A popular Tokyo aquarium said Thursday it has resumed partial operations following the death of more than 94 percent of the fish in its largest tank, most likely because of a lack of oxygen.

Sunshine Aquarium said 1,235 fish had died in the “Sunshine Lagoon” tank. Only 73 fish, including zebra sharks and stingrays, survived.

The aquarium said it plans to gradually add more fish to the 12-meter (40-foot) -wide tank.

Underwater performances in the tank and guided tours of the aquarium have been suspended.

A security guard found the dead fish on Wednesday. The aquarium said it closed that day as officials investigated the cause and cleaned up the tank.

It said the stoppage of a bubble-generating tank cleaner that also enhances circulation of chemicals for treating unhealthy fish might have caused an oxygen decline.

The aquarium has more than 500 species of sea creatures, including penguins, sea lions and otters.

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