It’s been awhile, thankfully, since the National Zoo had to answer questions about mysterious animal deaths. Unfortunately, it seems that 11 of the zoo’s 18 stingrays and two arowana fish died over the holiday weekend, the Post reports. After they were found Monday morning, immediate testing of the water revealed that levels of dissolved oxygen were too low in the Amazonia exhibit’s aquarium. The levels were fixed and the remaining stingrays are doing fine.