A fascinating study suggesting that koi will self-seek water at 90 deg F (32 C) in order to reduce the suffering of KHVD (cyprinid herpesvirus 3). This is very exciting research, but of course as noted by the researchers out of Belgium, a vaccine is still needed.
 
Abstract
The recently designated cyprinid herpesvirus 3 (CyHV-3) is an emerging agent that causes fatal disease in common and koi carp. Since its emergence in the late 1990s, this highly contagious pathogen has caused severe financial losses in common and koi carp culture industries worldwide. In addition to its economic role, recent studies suggest that CyHV-3 may have a role in fundamental research. CyHV-3 has the largest genome among viruses in the order Herpesvirales and serves as a model for mutagenesis of large DNA viruses. Other studies suggest that the skin of teleost fish represents an efficient portal of entry for certain viruses. Recent advances with regard to CyHV-3 provide a role for this virus in fundamental and applied research.
 
https://wwwnc.cdc.gov/eid/article/16/12/10-0593_article