Photo courtesy National Zoo/Jessie Cohen. Two adult arapaima fish have died in the Amazonia exhibit at the Smithsonian’s National Zoo, zoo officials said Monday. A third fish that also appears sick is being closely monitored, while a fourth has been started on antibiotics preemptively. Zoo officials said the first fish died Nov. 19, while the second died over Thanksgiving weekend, on Friday, Nov. 27. No conclusive cause of death has been determined, but the…