article_snippet: Unexplained:The cause of the deaths of more than 150 fish is being explored, but is not currently known .
article_snippet: The tank's lighting and other factors that could have caused stress among the fish, or the presence of a poisonous substance in the water are being considered.
article_snippet: In December, The tank's population suddenly started plunging with just 30 fish alive by mid-January.
article_snippet: An earlier examination has found some sort of virus among some of the dead fish, but it wasn't the kind that is usually fatal in fish farms.'
highlights_snippet: More than 150 fish died at an Japanese aquarium and only one survived .
  
highlights_snippet: Poisoning and factors that could have caused fish stress being explored .
  