I inherited about 15 Koi and I just built a pond. Its about 11' x 15', 5' in the deepest and 3' in the shallowest. I have a skimmer like box with a internal pump inside leading to a waterfall. My question is, living in Michigan, do I keep my pump running all winter long. Or do I shut things down, drain them, etc. & if so what keeps the pond oxygenated. Thanks for your help, this is my first pond!