According to the following report, it seems that Matsuda Yorijo's greenhouse in Mushigame, Yamakoshi was the victim of deliberate sabotage.
The drain pipe was pulled out causing the ponds to completely drain and killing some 1,000 koi 2-7 years old from 25-50cm (reported) as a result.
Original Japanese news article from Niigata Nippo below:
https://www.niigata-nippo.co.jp/news...106505790.html