wintering Koi fish

Question
We have a very small plastic insert pond in which we keep koi, but since we live in upstate NY it freezes over. We tried keeping the koi in the pond, but it died. This year we brought it in and used the pond water and conditioner and it still died.Any suggestions? I don't want to keep losing the koi. We have four little ones in a separate tank, which survived so far.  Should we add gravel and real plants? Thanks for your suggestions.
Anita

Thanks, Anita

Answer
I would bring them in and put them in an aquarium that has a heater, filter and air stone.... Feed them accordingly and do regular tank maintenance... I know this a generalized answer to your question... so let me know what you need help with and I will go more in depth... real plants always help.... gravel... its usually just for the fish keeper... dave