i'm looking for some help in wintering Koi in my pond in Calgary. It is about 1200 gallons and 3 feet deep. I have a home made skippy filter supplied with a submersible pump.I just hooked up a uv unit in line about 2 weeks ago to try it.I have kept all 7 koi (up to about 1 ft long) inside a 50 gallon tank in the past but would like to leave them out. Do I need to put in different filter when I shut down the skippy, and what about circulation,aeration and freeze protection? Any help would be appreciated.