umm, I know that you should be able to keep goldfish in there through the winter. but Koi get huge and Wouldnt like that shallow of a pond.
me putting a Koi in my pond was like putting a 8ft python in a 5gal tank lol.
yours is wide enough/long enough I just dont think its near deep enough! stick with gold fish they are easy and cheap to keep in there. what I would really like to do is fix something up so I could have turtles.. that would be nice!