A good basic read on KHV and how to recognize. 
 
Koi herpesvirus (KHV) is a highly contagious virus that causes significant morbidity and mortality in common carp (Cyprinus carpio) varieties (Hedrick et al. 2000; Haenen et al. 2004). Common carp is raised as a foodfish in many countries and has also been selectively bred for the ornamental fish industry where it is known as koi. The first recognized case of KHV occurred in the United Kingdom in 1996 (Haenen et al. 2004). Since then other cases have been confirmed in almost all countries that culture koi and/or common carp with the exception of Australia (Hedrick et al. 2000; Haenen et al. 2004; Pokorova et al. 2005). This information sheet is intended to inform veterinarians, biologists, fish producers and hobbyists about KHV disease.
 
VM-149/VM113: Koi Herpesvirus Disease (KHVD) (ufl.edu)