Koi are essentially a pond fish. They grow to around 24 inches long and were developed by the Japanese to have quite broad backs in proportion to the size of their body and be of specific colouring...
There are several possibilities, not limited to predetory birds, neighborhood pets, children taking them to play with, and many more!!!!!!! hope you find out which one and solve the problem!!!
As big as possible. Ideally 3ft to 6ft deep rectangular shape is best with no wider than 12 ft as this makes it harder to catch them if needs be. 18 to 23ft is good. Koi ponds can be smaller but this...