I have a hatchery tank that I am using for some small koi, and I made a simple K1 filter for it from a 5 gallon bucket. I have been using it for about a year and it works really well, and is very easy to clean & do water changes. I have pictures and explanations on these pages:
About the filter
http://www.shortypen.com/projects/po...lter/index.htm
Cleaning the filter and changing the water
http://www.shortypen.com/projects/pond/gbackf/index.htm