A popular Tokyo aquarium says 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 Thursday that 1,235 fish had died in the "Sunshine Lagoon" tank. Only 73 fish survived. The aquarium says it plans to gradually add more fish to the 12-meter (40-foot) -wide tank.

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.