Thanks Janet, actual depth is about 3.5 feet. How deep would I have to go?

Janet wrote:

If you're serious about keeping koi then I suggest you stop where you are
right now and head on over to the pond building forum at
http://www.koiphen.com There are some very knowlegable folks there that
really know their stuff. I hate to say it, but 3 feet deep isn't going to be
deep enough for koi. You're going to need some heavy duty filtration as
well... there is a reason koi are called pond pigs...
Janet in hot, humid Niagara Falls