strofn

The primary problem could be a virus such as the Koi Herpes Virus. It was responsible for a massive die off in New York in 2005. 2005 article. KHV affects the common carp (Koi were domesticated from them) and can look like other diseases. See KHV for details. The situation could have been caused by someone deciding they didn't like their pond anymore and dumping infected koi into Scugog.