First of all, I am not a serious koi keeper but I really like to add koi fish in my pond.
I was always wanted to have a natural shallow looking koi pond. But after doing research, I found that a minimum depth of a koi pond should be at least 3' deep. I decided to go with cheapy water garden pond 7 years ago. I recently watching Tom Burton koi pond (18" deep) on youtube (see link below) then I decided to have a koi pond that is about 30" deep with good filtration.