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 zoo suspects a bacterial infection.

Staff could find no apparent malfunctions with the aquatic equipment inside the Amazonia exhibit. The exhibit also includes catfish and pacus, all of which officials said appear to be in good health.

Arapaima are found in the Amazon in South America, and typically grow between six to seven feet in length. The approximate age of both the two fish that died is 17, which zoo officials said is relatively long-lived for the species in captivity.