A Tokyo aquarium suffering through massive fish deaths in its renowned tuna tank since late last year reopened the tank to the public Monday after adding 80 tuna.

After the facility closed Sunday, workers at Tokyo Sea Life Park in Edogawa Ward added 80 tuna to the huge doughnut-shaped tank, which used to be among the venue's most popular attractions and home to nearly 160 fish.

The cause of the tuna deaths has not been determined yet.

Researchers are studying a range of possibilities, including the tank's lighting and other factors that could have caused stress among the fish, or even the presence of a poisonous substance in the water.

The 2,200-ton, 30-meter-diameter tank once housed 69 bluefin tuna, 52 eastern little tuna and 38 oriental bonito. The tuna population started to decline in December and only one remained in the tank before the latest additions.

In the last few months, the aquarium has added other fish species to the tank in stages to see what happens. None of the other species suffered mass deaths, according to the aquarium.